Clu Gulager was born William Martin Gulager in Holdenville, Hughes County, Oklahoma. His nickname was given to him by his father for the clu-clu birds (known in English as martins, like his middle name) that were nesting at the Gulager home at the time Clu was born. He grew up on his uncle's ranch as a cowhand and when he was old enough he joined the United States Marine Corps for a stint from 1946-1948. He got the acting bug being in army plays so when he left he used the GI Bill of Rights to study acting. During this time he met his wife, actress Miriam Byrd-Nethery. They wed in 1952 and had two children: John (born 1957) and Tom (born 1965) The couple was married for more than 50 years until her death in 2003 from cancer.

Gulager's career started off as bit parts on popular western shows usually playing the heavy. Shows like Wanted Dead or Alive, Have Gun Will Travel, Laramie, Riverboat. He scored big with The Untouchables as "Mad Dog Coll", which led to him being offered the role of "Billy the Kid" on The Tall Man from 1960-1962, which also starred Barry Sullivan as "Pat Garrett". The show was pulled after two seasons reportedly because the powers that were didn't like kids seeing Billy the Kid as a hero.

His next big break was playing Deputy Emmett Ryker on The Virginian from 1964-1968. During this time he also fared very well as Lee Marvin's sidekick in the 1964 TV film The Killers, which was considered too violent for TV so it went to theaters. Having being burned out being a TV star he tried to break into films, mostly as a character actor. His stand out films were The Last Picture Show (1971, playing Ellen Burstyn's lover), McQ (1974) with John Wayne, and A Force of One (1979) with Chuck Norris, with whom he would later work in the 1990s on Walker, Texas Ranger.

Gulager was also cast in San Francisco International Airport, with Lloyd Bridges, which failed big time. Throughout the 1970s and 1980s he was in almost every show around, playing bit parts. Then the unthinkable happened: he found a second career as a horror film actor; he followed the footsteps of other TV actors who were stuck in TV hell, like Doug McClure (his costar from The Virginian) and Christopher George. Both men found new careers in B-movies and late night horror films. Gulager finally got a lead part in Dan O'Bannon's cult classic The Return of the Living Dead (1985). He also was in A Nightmare on Elm Street Part 2: Freddy's Revenge (1985).

Throughout the 1980s and 1990s, he appeared in TV and in the occasional horror flick. In 2005 he started acting in his son's horror films -- the Feasts movies and Piranha DD in his 80s. Not letting age get in his way, he was a horror fan favorite and still showed up at conventions at almost 90.