Marshall Eriksen is Ted Mosby's best friend and Lily Aldrin's husband. Marshall is a lawyer with a dream of practicing a law firm to save the environment, but is currently working in the corporate sector to earn some money for him and Lily.

Marshall met Ted Mosby and Lily Aldrin on the first day of their freshman year at Wesleyan University. He is a Columbia Law School graduate originally from St. Cloud, Minnesota.

In Season 4, he reveals that he is the slam dunk champion of Nicollet Country, Minnesota, with his trademark dunk the "White Windmill". St. Cloud is not in Nicollet County, and rather inent Stearns, Benton, and Sherburne counties, and Nicollet County is actually to the south, near St. Peter. He can no longer dunk because he has dancer's hip. Even though he's six-foot-four, he is the shortest male member of his family. A donkey supposedly had eaten Marshall's pants the last time he was in Trenton, and at Bill's bachelor party in Memphis, they had to pump out several nickels from his stomach.

Marshall often uses the word "lawyered" whenever he makes a point in an argument using facts and numbers. The term has become somewhat of a catchphrase for him.

Marshall is extremely loyal to his friends and in particular Ted, his best friend.

In the Season 1 Finale, his wedding plans are derailed when Lily pursues a painting fellowship in San Francisco, triggering a fight that ends their engagement. He and Lily rekindle their romantic relationship in Mid-Season 2, get re-engaged, and are actually married for 12 seconds in the episode Atlantic City. Their wedding takes place during the last two episodes of Season 2.

Marshall accidentally shot Lily in the eye with a champagne cork the night he proposed. Another time, he (also accidentally) stabbed her with a sword during an impromptu duel with Ted over that had the rights to the apartment after Marshall and Lily got married.

Marshall is very good at games, as shown in the episode Game Night. He always wins, which prompts his friends to put him in charge of game night with the exception that he can't play. Marshall took this to mean he must make up a game, and so he does: Marshgammon, which "incorporates the best parts of all other games" with the noticeable exception of backgammon. It is revealed in Season 1 that he, his brothers and his dad also play a made-up game every Thanksgiving: BaskIceBall, although Marshall states it's just an excuse for the men in his family to hit each other. When the gang was gambling in Atlantic City in the episode Atlantic City, Marshall was the only person to figure out the crazy Chinese game Barney was playing. In college, Marshall and Ted play a game called "Zitch Dog" on their road trip, as shown in Arrivederci, Fiero. Marshall won even while he was sleeping. Also, Marshall is an extremely good dancer, as seen in the episode Okay Awesome, he spent a few minutes in the bathroom looking for painkillers, and came out in an euphoric state.

Marshall has a great love of, and often preoccupation with, food. In episodes such as The Pineapple Incident and Drum roll, Please while other characters are curious about the details of Ted's love life, Marshall is more concerned about the origin of foods associated with these details. He eats very often, consuming food such as popcorn or ice cream as a result of emotional situations.