Joanna is very similar to her owner: mean, scary, entertaining, and more intelligent than for which she's given credit. She is very fond of eggs and enjoys messing with McLeach by swiping them. However, she is not as anthropomorphic as the other animals in the film.
In the movie, when Cody gets caught in one of McLeach's traps, McLeach and Joanna show up thinking they have caught an animal, but when McLeach sees that it was Cody, he tries to blame it all on Joanna, saying that she has buried a big hole in the ground. While McLeach helps Cody out of the hole, Joanna spots a mouse in Cody's backpack and jumps on Cody and causes McLeach to fall in the hole.
When McLeach gets out of the hole, he points his gun at Joanna, causing her to cower, but when he spots Marahute's feather in Cody's backpack, he congratulates Joanna on what she has done and tries to get Cody to tell him where Marahute is, but Cody rejects and runs off and Joanna chases him, trapping him at the edge of Crocodile Falls. When Cody tries to get out his pocket knife to defeat Joanna with, McLeach grabs him, and both he and Joanna kidnap the boy and take him to their abandoned opal mines home.
At the opal mines, Joanna takes a bath while McLeach ties Cody up and tries to get him to tell him where Marahute's nest is, but Cody refuses and McLeach angrily kicks his boiling pot on the fire, causing some of the ashes of the fire to fall in Joanna's bath and causing Joanna to look up at her master nervously. McLeach puts Cody in a cage along with some other animals to think about telling him where Marahute is, and he orders Joanna to listen if there's any disturbance from the animals or Cody.
When Cody and the animals try to get the keys to escape, Joanna comes back and takes the keys away, and she puts them up on the hook so Cody and the animals cannot escape. The next day when McLeach goes to get some eggs for his breakfast, Joanna secretly follows him and she secretly eats all his eggs, causing McLeach to get angry, but just as McLeach is about to hit Joanna with the egg box, he figures out that Marahute's eggs are Cody's weak spot. Then Joanna hears more noise from the animal prison room and learns that one of the animals named Frank the lizard has managed to get the keys, and she starts chasing Frank who manages to throw the keys to Cody and jumps on McLeach's gun, causing the gun to go off and push Joanna into a box nearby. But just as Cody is about to free the other animals, McLeach comes and both he and Joanna take the boy outside, and McLeach lies to Cody saying that Marahute has been shot by another poacher and orders Cody to leave.
As Cody leaves, McLeach and Joanna secretly follow him in McLeach's truck but to their unknown Bernard, Bianca, and Jake hop onto the truck and go with the villains to follow Cody. At the nest, McLeach manages to capture both Marahute, Cody, Jake, and Bianca in the cage in the back of his truck and orders, Joanna, to eat Marahute's eggs so the eagle will stay rare. When Joanna sees the eggs, she tries to eat them, but they will not go into her mouth and feel hard, so she knocks them off the cliff with her long tail, thinking they have cracked, and returns to McLeach, but it was revealed that those were actually rocks and Bernard, who was not captured by McLeach, managed to hide Marahute's real eggs safe from Joanna.
McLeach drives his truck to Crocodile Falls and ties Cody up to the crane, and prepares to feed the boy to the crocodiles, but the engine goes dead and a razorback jumps out of the truck, and McLeach orders Joanna to sniff around for suspicion. Joanna finds Bernard and she chases him and jumps on McLeach, and Bernard manages to push them both into the water. Joanna manages to survive by swimming onto a rock, and she waves goodbye to McLeach as he goes over the waterfall to his death. Presumably, she went on to live in the wild in her natural habitat.
Joanna makes her appearance in "House of Crime" alongside McLeach which was her only appearance in House of Mouse.
- Joanna had a compartment almost similar to a dog.
- Although Joanna is McLeach's pet, it is heavily implied that she is frequently abused by McLeach and that her following her master was primarily out of fear. However, she was particularly vicious with other animals, implying that she was used like a hunting dog by him.
- Monitor lizards in real life can be trained to hunt like dogs with consistent training, so this is not far-fetched.
- Frank Welker not only provided the voice for Joanna but also the voice for Marahute, another female animal character without human speech.
- Joanna is likely either a perentie or an Argus monitor lizard (both of which are called goannas in Australia). Both are exceedingly large, powerful, fast and intelligent monitors from Australia. However, given her size, it is likely Joanna is a perentie, which is the fourth-largest lizard in the world and the second-longest at 8 feet.