He is a pudgy young mouse who initially works as a janitor for the Rescue Aid Society, the mouse version of the United Nations. After accompanying Miss Bianca on their first mission, he is made the American ambassador for the Society.
Bernard is somewhat cautious and superstitious. He also suffers from triskaidekaphobia or fear of the number 13. However, he has also proved to be very brave and courageous. Aside from that, despite being a small mouse, Bernard has shown himself to be incredibly strong. His most notable feats of strength are grabbing a razor back by the tusks and toppling it over, pushing McLeach (with Joanna in tow) off a cliff and holding onto Cody by a rope to prevent him from falling off killer rapids. His partner also makes jokes about him suffering from Acrophobia, as she calls him a "fraidy cat" before they board Orville for flight.
Bernard works as a janitor for the Rescue Aid Society within the United Nations Headquarters in New York City. When a message in a bottle arrives, Bernard is tasked with retrieving the note from inside. He climbs up a comb-ladder, during which he reveals his triskaidekaphobia. He is able to get the note out but falls in the bottle. Upon learning that a girl named "Penny" is in trouble, the Hungarian agent, Miss Bianca, asks for the assignment. The Chairmouse agrees, but Bernard points out that it is too dangerous for her to go alone. The Chairmouse agrees and suggests that she take along a co-agent. After some thought, Bianca chooses Bernard as her co-agent (much to the surprise of the Chairmouse), despite him being a janitor.
Bernard and Bianca's first stop is Morningside Orphanage - the place that Penny's note was addressed to. There, they learn from Rufus the Cat that Penny had been depressed from being passed over for adoption, and had disappeared a short time afterward. Rufus then recalls that right before she had disappeared, a strange woman had offered Penny a ride. Rufus tells Bernard and Bianca that the woman runs a nearby pawn shop with her partner.
Bernard and Bianca then travel to the pawn shop. They find Penny's schoolbooks, signaling that Penny had been there. They then overhear Medusa talking to her partner, Mr. Snoops. Bernard and Bianca learn that that Penny had been taken to Devil's Bayou, apparently to look for a diamond. After learning that Penny had been sending messages in bottles, Medusa declares that she will be flying out there herself.
The two mice decide that they must go along with Medusa, so they stow away in Medusa's suitcase as she carelessly packs. However, due to Medusa's reckless driving, the car (a convertible) makes a sharp turn on the street and the suitcase falls out, leaving Bianca and Bernard stranded.
The next day, Bernard and Bianca prepare to fly to Devil's Bayou on Albatross Airlines. They arrive late, causing Bernard to believe that they missed the flight. However, the pilot soon radios in asking for permission to land, revealing that he had been running late. Bernard gives him permission and soon sees a large Albatross making a crash landing onto the runway. After the albatross, named Orville, calls the landing one of his better ones, Bernard tries to convince Bianca to take the train. However, Orville quickly offers to take them to the bayou. While boarding, Bernard realizes that the ramp has thirteen steps; Bianca encourages him to jump the last step. Bernard then reads the pre-fight checklist (which leaves Bernard concerned for both his and Bianca's safety) and the group is soon flying to Devil's Bayou.
Right before landing, Orville is shot at by fireworks and the group ends up in the water. They are helped by two swamp mice, Luke and Ellie Mae. Bianca and Bernard soon spot two crocodiles with Penny. Ellie Mae recruits Evinrude, who is said to have the fastest boat around, and Bernard and Bianca follow the crocodiles to an abandoned riverboat, which serves as Medusa's hideout. Once aboard, Bernard and Bianca learn that Penny was kidnapped to find the Devil's Eye, the world's largest diamond. Penny is the only one small enough to fit down the entrance to the cavern where it is hidden. When Medusa's crocodiles discover the mice (Bernard realizes that they smelled Bianca's perfume), Bernard and Bianca are forced to flee. They escape with Evinrude's help and are able to find Penny in her room. After Bernard and Bianca introduce themselves, the three form a plan to escape. Medusa's crocodiles would be locked up in the elevator, while Medusa and Mr. Snoops are distracted by fireworks. Meanwhile, Penny, Bernard, and Bianca would escape on Medusa's Swamp Mobile. To this effect, Bernard sends Evinrude to get the Swamp Folks to assist.
When Penny is forced back down the hole for the diamond, Bernard and Bianca stow away in Penny's pocket. With the help of Bernard and Bianca, Penny is able to find and retrieve the diamond from a pirate's skull. Back above ground, Medusa takes the diamond and chooses to hide it in Penny's teddy bear. Penny is carried away by Medusa's crocodiles, but Bernard and Bianca are able to find some rope and follow Medusa to the riverboat. At the riverboat, Bernard and Bianca trip Medusa with the rope, allowing Penny to grab her teddy bear. The Swamp Folks quickly arrive and the plan is put into motion. Bernard, Bianca, and Penny are able to escape when Medusa's crocodiles turn on her.
Back in New York at the U.N., Bernard and Bianca watch a television program, which is telling Penny's story. The reporter reveals that Penny was adopted and the excited Penny thanks Bernard and Bianca on camera, much to the reporter's confusion. The end of the film reveals that another call for help has come in. Bernard and Bianca accept it after Bianca volunteers them both (ironically, on Friday 13).Cody, who was kidnapped by a poacher named McLeach in Australia.
When Bernard and Bianca go to Albatross Airlines to get a flight to Australia, they learn that Orville's brother, Wilbur, has taken over the business and beg him to take them but, at first, Wilbur rejects until Bernard and Bianca tell him about the kidnapping and he gives them the flight but tells them "they're gonna have to make connections with a bigger bird". They sleep in a plane's cargo bay and the next day they arrive in Australia and Wilbur flies out to Bernard's nervousness and Bernard asks Bianca if they can take the train from now on. When Wilbur asks a kangaroo mouse named Jake's permission to land, Jake (over the radio) says no because the runway isn't big enough for a bird of Wilbur's size and Bernard tries to get Wilbur to listen to Jake but Wilbur doesn't and lands on the runway. While Jake helps Bianca off, Wilbur offers to take Bernard's bags, but then has a problem with his back and gives Bernard his bags back.
After Bernard and Bianca take Wilbur to the hospital, Bernard tries to read the map in order to find McLeach's hideout, but Jake tells him that a map isn't good in the outback and offers to guide Bernard and Bianca to McLeach's Lair. While riding a wombat, Bernard gets accidentally thrown off into a briar patch. While Jake is gone, Bianca helps Bernard get the briars out of his back and Bernard tries to propose to her again, but then Jake shows up with a snake named Twister that he had tamed, and Bianca hops up on top of Twister's head with Jake while Bernard sits sadly on the back of Twister's back.
That night, the mice continue their travel on fireflies, during which Bernard has more trouble controlling the firefly he's riding on.
The next day, Bernard, Bianca, and Jake arrive at McLeach's hideout but, when they try to get in, the door opens, and the mice watch on top as McLeach throws Cody out and tells him that his bird friend named Marahute has been shot and that her eggs are alone. When Cody leaves, McLeach secretly follows in his truck, but Bernard, Bianca, and Jake manage to climb aboard and follow, too. When Bernard, Bianca, and Jake arrive at the cliff, they go down to the nest and try to warn Cody about McLeach, but then Marahute shows up and McLeach captures her, Cody, Bianca, and Jake. Bernard manages to stay hidden and hides Marahute's eggs from McLeach's pet lizard, Joanna and puts rocks in the eggs' place and he manages to fool Joanna when she comes to eat the eggs. After Joanna leaves, Wilbur shows up and Bernard forces him to watch the eggs while he goes and rescues Cody, Marahute, Jake, and Bianca from McLeach. Along the way, Bernard gets a ride from an old razorback after taming it with the same method Jake used on Twister.
When Bernard finally reaches McLeach's truck at Crocodile Falls, he manages to steal the keys from the truck and hands them to Jake and Bianca, who are trapped inside the cage with Marahute. Joanna spots Bernard and gives chase, but Bernard (who had spotted McLeach trying to shoot at the rope that had Cody tied to it) jumps into McLeach's pocket, causing Joanna to jump on McLeach and Bernard jumps out of the pocket and manages to push the two villains into the water, sending McLeach over the waterfall to his death. Then the rope that has Cody tied up breaks and Cody is in the water, but Bernard manages to hang onto the rope and, just as he and Cody are about to go over the waterfall as well, Jake and Bianca swoop down on Marahute and rescue them. After Cody thanks Bernard for saving him, Bernard proposes to Bianca and she agrees to marry him, and Jake gives Bernard the thumbs up. The film ends with the whole company flying to Cody's house on Marahute.
Bernard, along with Bianca, make appearances in the TV series House of Mouse. Bernard also appears in Mickey's Magical Christmas: Snowed in at the House of Mouse and Mickey's House of Villains. Bernard's most notable appearances are his background cameos in various episodes.