Einstein is a gray Great Dane, whose personality is a play on the stereotype of Great Danes being friendly but silly and his name is also a play on this. He is the largest of all the dogs in the movie, larger even than Roscoe and DeSoto. He is protective of Tito and all of his friends. It is suggested that Einstein is not a great fighter despite his size and endurance, mostly because of his amicable nature, lack of intelligence, and age, since he is reluctant to take on Roscoe even after telling him to pick on somebody his own size. He is kind, friendly, silly, shrewd, funny, dim-witted, and protective.
Einstein is first seen on Fagin's house boat sleeping with a broken tennis racket in his mouth as his "loot" for the day until he is awakened by Tito jumping on his nose as he goes to add his "loot" to their collection for Fagin. Einstein tells Francis what he got as a "loot." Francis mocks him, telling him that all they need now is the court and the net, but Einstein takes him seriously, as he inquired as to whether the houseboat was big enough. Einstein is then told by Tito about his loot. Later, Einstein along with the rest of the dog-gang are upset by the fact that Francis forgot to get the food today since it was his turn.
When Dodger enters the houseboat, he introduces Einstein and his friends to their dinner: hot dogs. When Dodger is asked about how he got them, he mentions every event occurring including "the hot dogs are sizzling" which amuses Einstein with the fact that he "loves a story with food in it." As soon as Dodger mentions his "monster" (Oliver) in the story, Einstein becomes a bit scared and even more spooked like the gang when the "monster" falls in. Einstein is later seen running and hiding in fear in a trash can but when the "monster" turns out to be a cat (Oliver), he's calm. The gang (except Dodger) surround Oliver and ask him questions about how and why he came to their place, with Rita snarling at him, "Now how'd you find this place, cat?!". Oliver says he followed Dodger, Tito thinks he's lying while repeating "he's lying" countless times and is kicked by Rita to shut him up before Francis asks: "Now why would a cat follow a dog?" to which Einstein replies: "Yeah?" Then, Oliver reveals to them the truth about his involvement in Dodger's scheme of hot dogs theft. Dodger admits to his friends that Oliver is telling the truth and the gang along with Dodger begin to develop respect for him. The gang starts to mock Dodger's story resulting in Dodger starting a "fight" with Tito then Francis, to which Einstein joins in enthusiastically, and then Rita after Tito bonks her in the side of her head.
When Fagin, the dogs' owner, enters and tells them to stop the "fight" he is noticing, the dogs notice the dog biscuits Fagin bought them and then they run and jump on him to give him a warm welcome home, mostly Einstein, who repeatedly licks Fagin right in the face with his giant pink tongue despite Fagin's protests.
When Roscoe and DeSoto enter their home after hearing a honking noise, Fagin goes up to have a meeting with their master and his evil boss, Bill Sykes, whom he owes money to, while Roscoe taunts the gang and DeSoto sniffs around the place. Tito attempts to fight Roscoe after Roscoe tries to flirt with Rita and threatens Francis. Einstein, knowing Tito would not have a chance against Roscoe, holds the over-eager Chihuahua back. When Roscoe, amused by Tito's fiery temper, insists that Tito be set loose, Einstein suggests that Roscoe pick on somebody his own size, but being an amicable and unintelligent dog, does not really have the savagery or cunning to take on either of the Dobermans, despite his size, which Roscoe makes clear when he retorts to Einstein's challenge with "Like you, old man?!". However, he joins in with the other dogs to protect Oliver from both of them after Oliver had scratched DeSoto's nose while DeSoto was intending to eat him once it was time for the two evil dogs to leave when hearing their master calling them with his car horn.
When Fagin enters home, after the Dobermans leave while swearing revenge on them, he's all wet after falling off a pier and full of sadness since he's been given three days to pay Mr. Sykes back or else there's no telling what will happen to him, his house, and his dogs, the dogs do everything they can to cheer him up, mostly Einstein when giving him a dog biscuit to eat. Fagin is rejuvenated and thanks them, to which Einstein replies with some more love and affection, mostly licking. When it's time for bed, Einstein insists Fagin reads them a bedtime story. At first, Fagin is reluctant because he's too tired, but when he hears and sees Einstein whining sadly, Fagin accepts "but just one chapter." While reading, Einstein is seen panting happily and quickly on Tito's hair, much to Tito's annoyance. Later, Einstein is seen sleeping with Oliver on Fagin's lap after story time's over.
The next day, the gang are riding in Fagin's scooter but then, they're told to get out on the street to search for food while Fagin searches for a way to earn or even steal money to pay back Sykes. Dodger intends to help Fagin with his financial problems and the gang agree to help too. While Rita sings "Streets of Gold," a vicious alley dog begins to chase Oliver, not before Dodger, Einstein, and the rest of the gang growl at the alley dog and shoo him away. Oliver even tries to "bark" like a dog to shoo the alley dog away himself and the gang find it amusing a bit. When Dodger notices a limousine, he comes up with a plan on how to steal its radio and give it to Fagin to exchange it for some money as partial payment to his debt with his evil boss. Later on, Einstein demonstrates one reason for his dumbness - he deliberately runs into Jenny Foxworth's limousine head first to make it stop, as part of Dodger's plan (while Francis plays the part of the dog hit as a way of distracting Winston, the driver and Foxworth family's butler while Oliver and Tito mostly handle the theft), and comes out of the collision dazed but otherwise fine. When the plan backfires, Tito is seen "flying" out of the car, making pinball noises from the electrocution, before the electric current flows through a street lamp, releasing its light bulb, which lands on Einstein's head, regaining his consciousness as the car drives away. Einstein and the gang get back to Fagin while Dodger and Tito follow the car where Oliver was taken in.
A few moments later, Dodger and Tito return to Fagin and the gang to inform them about everything, thinking their friend, Oliver, is in "danger" and that he's being "tortured." The gang decide to "rescue" Oliver, but Einstein asks "But what about Fagin?" as Fagin is seen trying to pawn a broken watch to the pawn shop for money, only for it to fall apart, leaving the pawn shop owner not amused by Fagin's attempt to earn money with broken items. The gang agree that they should be helping Fagin nevertheless. However, they agree to "help" first Oliver, then Fagin. Dodger discusses with the gang on how to perform the "rescue mission" the next day.
The next day, while Jenny, Oliver's new wealthy owner, is at school, the gang notice Winston banging on something with a rolling pin, thinking he's "torturing" Oliver. Einstein distracts Winston by ringing on the doorbell. Being the dumb dog he is, he just stays next to the doorbell and does nothing. Luckily, Dodger calls him in order to go and hide. After Francis distracts Winston "again", the dog gang are in the house while Winston is left outside. In the house, the gang's admiring the place more than focusing on their cat friend. Luckily, Dodger helps them snap out of it right away. After checking out the house, Dodger enters Georgette's (the Foxworth family's spoiled pedigree poodle dog show queen) room. Georgette panics and calls for Winston countless times, thinking that he's after her. When Dodger assures her that he isn't, Georgette feels insulted thus, giving him an uninteresting education about herself. Then, the rest of the gang enter her room thus, causing a mess in it, mostly Einstein when he sniffs her powder and sneezes, thus causing her to call Winston again, but when Dodger mentions that they'll leave as soon as they get their cat back, she's cooperative. Before cooperating with the "rescue," Winston manages to get back inside and checks to see if Georgette is alright. Einstein, along with Francis, are seen hiding under Georgette's bed cover with half of Georgette's body sticking out, Dodger and Rita posing as dog statue replicas, and Tito hanging on a coat hanger of the door. Winston finds it a bit weird, but nevertheless, he leaves. When Georgette takes the gang to Jenny's room, they notice Oliver sleeping happily and calmly on his pillow. Rita's the first one convinced he's okay and proposes they "forget the whole thing" but Georgette, eager to get rid of him, convinces them that "he's so traumatized" and then, hands him to them in a pillow sack held by Einstein and Francis and allows them to escape through the fire escape.
After Oliver's "rescue," and it turns out that he wanted to stay with Jenny, Einstein is confused by Oliver asking why they took him away, saying: "We rescued you, kitty. We, we brought you home." Later, when Dodger, his feelings hurt, insists he should go, Einstein says: "But he just got here." Einstein and the rest of the gang are sad to see their friend go and Oliver leaves, but is also sad and full of sorrow. Then, Fagin comes and takes Oliver back in. When Fagin notices Oliver's new golden collar tag, he has an idea for ransoming him.
After Fagin, Oliver, and Dodger's rendezvous with Sykes and his Dobermans, Einstein and the dogs are seen waiting with Fagin and Oliver, in his pocket, for their rendezvous with Oliver's unknown wealthy cat owner (Jenny). After Fagin and Jenny have a small conversation with Sykes and his dogs unnoticeably watching them in his car, Jenny turns out to be Oliver's owner with only a piggy bank as payment to get Oliver back and begins to cry. Fagin at first has a hard decision about whether to follow his good heart or not then, he gives up and pretends to find Oliver in a box and hands him to Jenny. Unfortunately, Sykes bursts into the scene, kidnaps Jenny, throws Oliver out of the window, and tells Fagin to keep his mouth shut and to consider their account closed.
Then, Dodger and the gang come to Oliver's aid. Dodger assures Oliver that they'll "absitively" get Jenny back. The dogs run to Sykes' warehouse without riding in Fagin's scooter this time. After an inspection of the warehouse has been made by Francis and Tito, it turns out everything is locked up so now, Dodger comes up with a plan on how to get inside. As the plan is set with Francis as the first "cannonball," Oliver as the second "cannonball," and Tito as the "maestro," Einstein and the gang call for Francis to see if he's ready and he is, thus jumping on the seesaw and launching Oliver up and through a window and inside. After Oliver opens a hatch from the inside, everyone enters in as well. The gang do everything they can to avoid their enemies: Roscoe and DeSoto (who were alerted by the noise Oliver made when entering the place and later, by Georgette's scream because of a broken nail) and security cameras (taken care of by Tito, though not getting electrocuted for his trouble again).
From the top of Sykes' office, Oliver and the dogs observe Sykes talking on the phone to Winston for the ransom from Jenny's parents, and Jenny okay, but tied up. Dodger comes up with a plan on how to distract Sykes and help Jenny. Tito, Francis, and Einstein distract Sykes by posing as a pizza delivery guy and luring him away, while Oliver and the rest of the dogs enter and try to rescue Jenny. Oliver and Jenny are happily reunited. The rest of the gang run as fast they can to avoid Sykes until they end up in front of Roscoe and DeSoto. When all seems lost, Georgette traps the evil dogs in a heavy tarpaulin and the gang return to their friends.
Chewing the ropes off Jenny fails and their enemies return, not before Dodger has another idea to get them out of here with a crane and Tito, first reluctant then, cooperative with Georgette's persuasion, as the mechanic. They manage to escape with Jenny before Sykes and his dogs break down the door, which the gang locked up earlier. Things go well until Sykes cuts off the controls of the crane with a fire axe, causing Oliver and the rest to fall and land on a long slide. They slide until they reach the end where, unfortunately, their enemies have them cornered.
When all seems lost, and before Sykes can snap his fingers to order his dogs to attack, they hear a honking noise, and breaking in through the window, it turns out to be Fagin with his scooter. Oliver and everyone else jump in and drive away as fast as they can. They seem to be able to outrun the two evil dogs, but not before Sykes manages to catch up with them in his Cadillac. Fagin, having no choice, drives his friends into a subway station but Sykes, driving like a madman, is able to drive in as well. Fagin then drives them through a tunnel with Sykes still in pursuit. When Sykes' car collides with Fagin's scooter, Jenny falls and ends up on Sykes' hood. Einstein and the rest of the dogs are seen barking and growling at their enemies, sans Georgette, while Oliver helps Jenny, later Dodger, and Dodger helps Oliver, killing Roscoe and DeSoto. On the Brooklyn Bridge, when Jenny is calling for Fagin's help, Fagin, with Einstein holding him, comes to help her while Tito takes command of his scooter. Sykes manages to grab Jenny's leg and pull her back in, not before Oliver and Dodger jump on him and fight him off, allowing Jenny to jump to Fagin on his scooter.
In front of them, a train is coming their way. Tito is capable of saving everyone by jumping on one of the cables of the bridge and driving them to the top, but Oliver and Dodger are still in the car fighting Sykes, who throws Oliver and Dodger out of his car before he notices the train heading his way and the train kills him, thus sending him and his car into the East River. When the rest of the company drive back down, Jenny runs to search for Oliver, only to find a sad Dodger approaching, holding a "thought to be dead" Oliver. At first, everyone thinks he's dead but when Jenny hears a soft meow from Oliver, she quickly realizes that Oliver is alive and everyone is rejoiced, except Georgette, who is still in great shock from the chase. Then, Dodger and the rest of the gang are accepted as Jenny's new best friends.
The next day, at Jenny's home, Oliver and his friends are celebrating Jenny's 8th birthday and giving her birthday gifts. When it's time to go, the gang (except Tito) say their good-byes to Oliver and vice versa.
Einstein is finally seen joining in with his friends at the end of the reprise of "Why Should I Worry" and driving in Fagin's scooter back home.
- Being a Great Dane which is considered to be friendly and unintelligent, Einstein is dumb and friendly and yet he was ironically named after Albert Einstein, a German Nobel Winner Genius.
- Einstein's gray spots and the bottom of his paws form a Hidden Mickey throughout the film.
- During Jenny's birthday, Einstein appears to be sitting on her left but then appears to be sitting on her right the rest of the way seconds later.
- Despite Tito being the only dog to continuously call Francis; "Frank" or "Frankie," Einstein and Rita were first heard calling him Frankie once when they were disappointed to find out that he forgot it was his turn to get the food for them the day Dodger met Oliver.
- Einstein says to Roscoe: "Why don't you pick someone of your own size?" is the same line Simba says to Shenzi in The Lion King since the two were protecting a smaller friend. (Zazu for Simba, Tito to Einstein.) In both cases, the villain's response is "Like you?"