Mark has red hair and, thanks to the army, now has muscles. He is a second lieutenant in the army, and the last time we saw him, he was home from active duty due to an injury. Liz calls her brother "Red," and Mark calls Liz "Carrot." One of Mark's favorite foods is cinnamon raisin bagels. When Mark was much younger, he had anger issues, even to the point of breaking a kid's nose over a game of checkers. However, Mark is no longer so temperamental. The last time we saw Mark, in A Lamb's Tale he was depressed. When he was is the army, he was in a building being bombed and his leg got injured, a fellow soldier and friend saw he was hurt and threw him out a window, but before hitting the ground, another bomb hit the building and killed his friend. But later in the episode, he talked with his deceased friend's father and the father comforted him and set Mark straight.
- Main article: Horton family
Mark is quite close to his sister Liz; both of them try to please each other whenever possible. In his first show, he was engaged to Natalie Horton, and it can be inferred from his second episode that they are now married.
When we last saw Mark, he was a bit of a broken man, due partially do the death of a friend in the war.