Full Name: Marcus John Philip Cassidy
Also Known As: Mister Mark
Birthday: June 7, 1992
Relatives: Hank Cassidy (father), Gloria Cassidy (mother), Butch Cassidy (younger brother), Peter Cassidy (grandfather)
Hair Color: Black
Eye Color: Black
Skin Color: Fair
Mark always wears a white polo shirt with an indigo collar and stripe, blue jeans, and white shoes.
- Mark shares his birthday with Welsh singer Tom Jones
- He also shares his birthday with North Irish actor Liam Neeson, best known for playing the character Qui-Gon Jinn in Star Wars: Episode I: The Phantom Menace
- He runs the repair yard on the Piperville Railway, which itself is run by his younger brother Butch after he inherited the railway from their grandfather
- He can play bass really well and owns a red Fender Jazz Bass
- Whenever Butch is using the brothers' shared Chevrolet Caprice, he uses a black 1958 Indian Trailblazer motorcycle as his main mode of transportation
- Whenever Butch is sick or injured, he takes over as controller of the railway until he (Butch) feels well again.