[2] After the United States entered World War II in 1941, Gordon enlisted in the U.S. Army, in which he served for two years and received an honorable discharge.

[2] In 1954 Gordon portrayed the outlaw Bill Doolin, a native Arkansan who founded the Wild Bunch gang and operated primarily in Kansas, on the syndicated television series Stories of the Century, starring and narrated by Jim Davis.

[citation needed] That same year Gordon was cast as Zip Wyatt in "Three Wanted Men" of Rex Allen's syndicated Western series Frontier Doctor.

He also played a gunslinging professional killer in the pilot for the television version of Gunsmoke; but many changes were later instituted on the series, such as the marshal's office and Long Branch Saloon looking markedly different and the relationship between Matt Dillon and Kitty being subtly more formal as well, so the episode was buried deep in the season in the hope that viewers would not notice, which apparently worked.

The film's director, Don Siegel, who worked with such screen tough guys as Clint Eastwood and John Wayne, related that "Leo Gordon was the scariest man I have ever met.

"[5][6] Other notable roles included that of John Dillinger in Siegel's Baby Face Nelson, opposite Mickey Rooney as the crazed protagonist.

Gordon may be most noted for his recurring Irish character Big Mike McComb on the ABC/Warner Brothers Western television series Maverick, working from 1957 to 1960 alongside James Garner and Jack Kelly.

Gordon's five appearances in the role include the much remembered episode "Shady Deal at Sunny Acres", "According to Hoyle" and "Plunder of Paradise" as well as "War of the Silver Kings", the first installment broadcast.

The screenwriters for the episode, Jim Fritzell and Everett Greenbaum, borrowed heavily from Gordon's brief criminal career and subsequent incarceration in order to supplement the role.

Another notable role was in the 1966 western The Night of the Grizzly opposite Clint Walker, one of the very few actors who could match Gordon's intense screen presence regarding physical size and strength.

Gordon played bounty hunter Cass Dowdy, who had a soft spot for his enemy's son but would, as one character said, "...hunt anything for a price, man or animal."

Gordon portrayed sympathetic parts when called upon to do so, including his performances in the Western Black Patch (1957), a film that he wrote, and in Roger Corman's civil rights drama The Intruder (1962), opposite a young William Shatner.

[citation needed] In 1970 Gordon appeared as Loma Bartender on The Men From Shiloh (rebranded name for The Virginian) on the episode titled "Hannah."

He also appeared in the film Maverick that same year with Mel Gibson, Jodie Foster, and James Garner.He starred in Tombstone Territory, Laramie and both acted in and wrote for Adam 12.