Max is first seen aboard Prince Eric's ship in the opening scene. He is still there when Ariel spies on Prince Eric's birthday party. While prancing around, Max catches Ariel's scent and tracks her down at her hiding place. Ariel gets scared and hides, but then glances back in curiosity and comes face to face with a panting Max. Immediately taking a liking to Ariel, Max licks her cheek affectionately before being called away by Eric. When Ariel comments on how Eric is very handsome shortly afterward, Scuttle gets confused, initially thinking she's talking about Max and says, "I dunno, he looks kinda hairy and slobbery to me".
Eric is soon presented with a birthday gift of a large statue portraying him in fancy attire and a heroic pose, but Eric subtly shows dislike for the portrayal, and Max openly growls at it in utter disapproval. When a storm hits and lightning sets the ship ablaze, everyone on board managed to evacuate except Max, who has been left aboard, trapped by the fire. Eric goes back to the ship and saves his best friend by having him jump into his arms to safety, risking his own life. Max later finds his master on shore after the shipwreck, licking his face happily. He is also able to smell Ariel behind some nearby rocks and spots her in the distance. Although he barks at her in attempt to get Eric and Grimsby's attention toward her, they ignore him.
Later, Eric is walking Max out on the beach. Max smells the now human Ariel further on down the beach and excitedly runs away from Eric toward Ariel, prompting Eric to follow the dog. He arrives and playfully chases a frightened Ariel up a rock, joyfully licking her face once more. Ariel wipes her cheek and grins at Max, realizing the dog means her no harm. When Eric arrives, Max gets frustrated when his master doesn't realize that Ariel is the same girl who saved his life.
As Eric is about to marry Vanessa (Ursula's human disguise), Max immediately realizes Vanessa's true colors, presumably able to see and/or smell the difference and growls at her as she passes by. Vanessa sharply kicks his face with the point of her high-heeled shoe to keep him quiet. Before the vows can be read, Scuttle leads a flock of bluebirds, amongst sea and lagoon critters, against Vanessa flustering and humiliating her so much that her hands are full. Max starts barking and jumping around seeking the opportunity to help with this battle, eventually breaking off his leash and running up behind the bedraggled bride and biting her bottom in revenge.
Vanessa forcibly lets go of Scuttle after Max bites her and he breaks her necklace off while she screams in pain and strains away from Max and the shell containing Ariel's voice breaks, freeing the voice, which is restored to Ariel, who has just arrived. Max then excitedly runs over to Ariel, leading Eric to her. Max is last seen at Eric and Ariel's wedding, where he happily licks both the bride and the groom in a congratulatory mood.
Max plays a small role in the sequel. He is seen at the beginning on board Eric's ship again; when Ariel takes him for a walk near the wall dividing the kingdom from the ocean, and at the end when he comes to help Eric rescue Melody, and later partakes in the swimming party, knocking Grimsby into the ocean while jumping in himself.
When Max growls at the Prince Eric statue, this snarling was reused from Toby from The Great Mouse Detective when he made stairs out of his left ear while he, Basil, Dr. Dawson, and Olivia were leaving Ratigan’s Lair.
Throughout the franchise, Max lived a remarkably long life for a dog, being a full-grown adult sheepdog at the time of the original film. Interestingly, he showed no signs of aging, and his physical condition had not deteriorated at all by the events of the sequel. In real life, the average typical lifespan of an Old English sheepdog is between 10 to 11 years.