He made many guest appearances on television series, his appearances ranged from The Rockford Files (episodes "Claire" and "A Deadly Maze"), Gunsmoke, Barbary Coast, Logan's Run, Police Woman, Battlestar Galactica, The Incredible Hulk (episode "The Antowuk Horror"), Wonder Woman, Buck Rogers in the 25th Century, The Dukes of Hazzard (episode "The Runaway"), T. J. Hooker, Tales of the Gold Monkey, Voyagers!, MacGyver, Simon & Simon, Sledge Hammer!, Star Trek: The Next Generation, Renegade and Crusade. He appeared on Land of the Giants in the first-season episode "Underground" as a police officer.
LeGault was born May 2, 1935 in Chicago, Illinois, the son of Mary Jean (née Kovachevich; died December 21, 1980) and Ernest Legault (1906–1941). LeGault's father, Ernest, was French-Canadian from Moose Creek, Ontario, Canada. LeGault's mother, Mary, was born in Illinois, the daughter of immigrants from the Austro-Hungarian Empire. The family was poor. He lived in an orphanage for a time between his father's death in 1941 and when his mother remarried in 1943. He started working at 11, and was fired from the railroad at 13 when they discovered he was not 18 as he had claimed.
LeGault's first three feature films were Elvis Presley movies, Girls! Girls! Girls! (1962) in which he was a stunt double for Elvis Presley, Kissin' Cousins (1964), and Viva Las Vegas (1964). He also appeared in Presley's 1968 NBC television special Elvis (also known as Elvis' 68 Comeback Special), where he sat at the side of the stage playing a tambourine. He appeared in Roustabout, another Presley film, as a carnival barker. In 1969, he appeared as Iago in the UK stage version of Jack Good's Catch My Soul: Rock Othello, and played Iago again in the 1974 Metromedia film version of Catch My Soul.
He starred in television series and in television movies and was known mainly for portraying military personnel, especially officers. His best known television role was in the 1980s series The A-Team as Colonel Roderick Decker—a United States Army colonel who tried to catch the fugitive Vietnam veterans. He played the role of Colonel Decker from 1983–86. He also had a recurring role in the other 1980s hit television series Magnum, P.I. as a United States Marine Corps colonel, Colonel "Buck" Greene.
Often playing stern colonels, the low-pitched, gravelly-voiced actor portrayed Colonel Glass in the 1981 comedy Stripes, starring Bill Murray and John Candy.
LeGault was on screen in a variety of programs including the short lived series Werewolf in 1987. In 1980, he starred with Kenny Rogers in the television movie The Gambler. He made a few appearances on the second season of Dynasty as gangster Ray Bonning. He appeared on Knight Rider in the pilot episode "Knight of the Phoenix" and appeared (as a different character) in the season 2 episode "Knight in Shining Armor"; and took on the roles of three different bad-guy characters in Airwolf (in To Snare a Wolf, Sweet Britches and Wildfire) as well as doing the voice-overs for the series' 1st Season "saga sell" teasers. He guest-starred on yet another hit 1980s television series Dallas as Al Halliday in 1989. In 1984 he also starred in Murder she wrote. Season 4 Episode 20
On the Knight Rider season 1 DVD pilot commentary, creator Glen A. Larson mentioned that Lance LeGault had "a voice that was four octaves lower than God's." This trait helped him obtain roles (often) as a villain or other "tough guy". It also resulted in a side career doing voice-over work. LeGault's trademark voice was at one point featured on self-guided tour cassettes at Elvis Presley's Graceland in Memphis, Tennessee. In the video game world, he was heard as the voice of Major Manson in the 1998 video game Battlezone II: Combat Commander.
He provided the voice of Junior the Buffalo in Disney's Home on the Range (2004). He also voiced Yank Justice in the nine-episode, 30-minute 1985 series Bigfoot and the Muscle Machines, part of Marvel Productions' Super Sunday block.
From 2009 to 2010, he performed voiceover work for Burger King, with the then-new "Angry Whopper" burger. He did voice-over work on commercials for Dodge and 7-Up as well.
LeGault died on Monday, September 10, 2012, at his home in Los Angeles at the age of 77, survived by his wife of 28 years Teresa, and their four children Mary, Teresa, Marcus and Lance.
LeGault's last role was in the 2013 movie Prince Avalanche, and the film is dedicated to him.