Marvin was born in New York City, the son of Lamont Waltman Marvin, an advertising executive and the head of the New York and New England Apple Institute and his wife Courtenay Washington Davidge, a fashion writer and beauty consultant. His father was a direct descendant of Matthew Marvin, Sr., who emigrated from Great Bentley, Essex, England in 1635 and helped found Hartford, Connecticut.

Marvin studied violin when he was young. As a teenager, Marvin "spent weekends and spare time hunting deer, puma, wild turkey and bobwhite in the wilds of the then-uncharted Everglades." He attended St. Leo Preparatory College in St. Leo, Florida after being expelled from several schools for bad behavior.

Marvin left school to join the United States Marine Corps, serving as a Scout Sniper in the 4th Marine Division. He was wounded in action during the WWII Battle of Saipan, during which most of his platoon were killed. Marvin's wound (in the buttocks) was from machine gun fire, which severed his sciatic nerve. He was awarded the Purple Heart medal and was given a medical discharge with the rank of Private First Class. Contrary to rumors, Marvin did not serve with Bob Keeshan during World War II.

After the war, while working as a plumber's assistant at a local community theatre in Upstate New York, Marvin was asked to replace an actor who had fallen ill during rehearsals. He then began an amateur off-Broadway acting career in New York City and served as an understudy in Broadway productions.

In 1950, Marvin moved to Hollywood. He found work in supporting roles, and from the beginning was cast in various war films. As a decorated combat veteran, Marvin was a natural in war dramas, where he frequently assisted the director and other actors in realistically portraying infantry movement, arranging costumes, and even adjusting war surplus military prop firearms. His debut was in You're in the Navy Now (1951), and in 1952 he appeared in several films, including Don Siegel's Duel at Silver Creek, Hangman's Knot, and the war drama Eight Iron Men. He played Gloria Grahame's vicious boyfriend in Fritz Lang's The Big Heat (1953). Marvin had a small but memorable role in The Wild One (1953) opposite Marlon Brando (Marvin's gang in the film was called "The Beetles"), followed by Seminole (1953) and Gun Fury (1953). He also had a small but memorable role as smartalecky sailor Meatball in The Caine Mutiny. He was again praised for his role as Hector the small town hood in Bad Day at Black Rock (1955) with Spencer Tracy.

During the mid-1950s, Marvin gradually began playing more substantial roles. He starred in Attack (1956) and The Missouri Traveler (1958) but it took over one hundred episodes as Chicago cop Frank Ballinger in the successful 1957-1960 television series M Squad to actually give him name recognition. One critic described the show as "a hyped-up, violent Dragnet... with a hard-as-nails Marvin" playing a police lieutenant.

In the 1960s, Marvin was given prominent co-starring roles in such films as The Comancheros (1961), The Man Who Shot Liberty Valance (1962) (as the title villain), and Donovan's Reef (1963), all with John Wayne. Marvin also guest-starred in Combat! and The Twilight Zone.

Thanks to director Don Siegel, Marvin appeared in the groundbreaking The Killers (1964) playing an organized, no-nonsense, efficient, businesslike professional assassin. The Killers was also the first movie in which Marvin received top billing and the only time Ronald Reagan played a villain.

Marvin won the 1965 Academy Award for Best Actor for his comic role in the offbeat western Cat Ballou starring Jane Fonda. He also won the Silver Bear for Best Actor at the 15th Berlin International Film Festival.

Following roles in The Professionals (1966) and the hugely successful The Dirty Dozen (1967), Marvin was given complete control over his next film. In Point Blank, an influential film with director John Boorman, he portrayed a hard-nosed criminal bent on revenge. Marvin, who had selected Boorman himself for the director's slot, had a central role in the film's development, plot line, and staging. In 1968, Marvin also appeared in another Boorman film, the critically acclaimed but commercially unsuccessful Hell in the Pacific, co-starring famed Japanese actor Toshirō Mifune. He had a hit song with "Wand'rin' Star" from the western musical Paint Your Wagon (1969). By this time he was getting paid a million dollars per film, $200,000 less than Paul Newman was making at the time; yet he was ambivalent about the movie business, even with its financial rewards: