- "She thinks too highly of herself. She should've kept a better look-out."
- ―Toby talking about Mavis, Bulldozer
Mavis is a young diesel engine who is full of her own ideas. She works at the Ffarquhar Quarry.
Mavis worked at the Ffarquhar Quarry Company to shunt trucks in their proper sidings for Toby to take to the harbour. After coming to a standstill in a frozen mud bank and rescuing Toby from a precarious tight-rope walk, Mavis was allowed to venture further down the line.
Occasionally, Mavis has been dispatched to help out the other various Quarry Companies on Sodor, especially with Bill and Ben.
Mavis was allowed to take trucks to Knapford, but did not listen to Toby's advice and collided into a rock-slide caused by a nearby factory. Her driver joked that she was by no means a "bulldozer" and could not possibly push the rocks aside.
Mavis frequently brings stone trucks to The Little Western Extension Project from Ffarquhar. On one occasion, while Scruff was working there, she brought some stone onto the bridge to strengthen the supports. Scruff, unaware that Mavis had gone onto the bridge, was shunting the breakdown crane to aid in some repair work. Unexpectedly, the crane swung sideways and knocked Mavis precariously over the edge. Thankfully, Oliver came to the rescue, and helped guide Mavis' wheels back onto the rails.
Despite almost falling over the bridge, Mavis was resilient and kept up with her stone trains to the Extension. However, when a disastrous windstorm came to Sodor, Mavis, BoCo, and Murdoch made haste to evacuate the area. After clearing the bridge, the foreman inquired where Scruff had ended up. Horrified, Mavis urged everyone to go back for the little tank engine. BoCo was the only one who followed, and eventually, the two diesels found Scruff buried under the roof of a shed destroyed by the violent twister. After that, Mavis, Scruff, and BoCo hurried to the bridge to be clear of the cataclysm, but the section split into two, and they had no choice but to make a jump. On Mavis' attempt, her wheels do not grip the rails and she landed sideways. Scruff returned with the breakdown crane to help Mavis, and Harold the Helicopter tries to pull the bridge into allignment so BoCo can cross, but it is far too late. The tornado made its way over, and brought down the bridge, taking Mavis with it in a fiery blaze. Many believed Mavis and her driver had been killed, and The Fat Controller held a special service for her at the sheds where he spoke to the others engines a eulogy of her brave efforts to save BoCo and Scruff. However, Mavis and her driver had indeed survived, being protected by the falling debris that shielded them from the flames. Oliver and BoCo were overjoyed to unearth this development, and Mavis was very thankful!
Mavis is a friendly, cheerful, young diesel. She can be a bit of an individual and prefers not to listen to outside advice, but she can be very charismatic and thoughtful about the welfare of her friends. She can also be wise and motherly at times.
- Season 1: Lift Bridge (cameo), Swagger and Swerves (cameo), Dodge (remastered version only cameo), Bon Voyage (cameo), Bulldozer, Scruff's Scaffolding, Conspiracy Theory (cameo)
- Season 2: Paxton and Norman (cameo), Mavis and the Tornado, Rock-Star, Munitions (cameo), Blunderbuss (cameo)
- Season 3: Percy and Asbestos (cameo)