Gainey was born in Jackson, Mississippi. In the early 1970s, he attended the American Conservatory Theater in San Francisco, California with Ken Hixon. Prior to this, he attended the University of Southern Mississippi. He worked as a mortician's apprentice before he decided to study acting. He served in the U.S. Army and was the company mailman at his post in Chakmakli, Turkey in 1969.

In 1981, he made his big-screen debut in Herbert Ross's musical Pennies from Heaven starring Steve Martin and Bernadette Peters. Since the early 1980s, he has been in over 50 films, including Two Idiots in Hollywood (1988), The Mighty Ducks (1992), The Fan (1996), Breakdown (1997), Con Air (1997), Terminator 3: Rise of the Machines (2003), Sideways (2004), Are We There Yet? (2005), The Dukes of Hazzard (2005), Wild Hogs (2007) and Mr. Woodcock (2007).

In 2003, Gainey appeared in The Last Cowboy, playing the role of Lance Henriksen's friend, along with Bradley Cooper and Jennie Garth. In 2004, Gainey appeared as a cuckolded husband in full frontal nudity in the award-winning arthouse film Sideways. He played Kurtz / The Wizard in the controversial road-revenge short film Apocalypse Oz. In 2007, he appeared in Mr. Woodcock, Wild Hogs, and Unearthed. In 2010, he appeared in Love Ranch and voiced the Captain of the Palace Guard in Tangled. In 2012, he appeared in Quentin Tarantino's film Django Unchained.