He was born John Sanford Posey in Hartford, Connecticut.
He graduated from the University of Florida in 1981. With a partner, he formed the comedy group, "Comedia", in Atlanta, Georgia in 1985 and moved to Los Angeles two years later.
He was originally set to play Danny Tanner on Full House. However, after the filming of the original pilot episode, Posey was replaced by Bob Saget.
Despite losing his spot on Full House, Posey has a extensive line of credits in his acting resume.
He has appeared in films like Manhunter, Robocop 3, Perry Mason: The Case of the Defiant Daughter, W.M.G, and the miniseries From the Earth to the Moon as astronaut John Young.
He has also appeared on numerous television shows for decades like Cheers, Dallas, Seinfeld, JAG, ER, 24, NCIS, Criminal Minds, Bones, Boston Legal, and Doc. However, Posey's notable television role are Jack McGinnis from My Wildest Dreams and Bill Millstone on How to Get Away with Murder.
For twelve years, he traveled throughout the country performing his critically acclaimed one-man show, Father, Son & Holy Coach. It's the tale of a father who wants a do-over in life and is determined to do so vicariously through his son. Posey portrayed multiple characters during the production.
John also works in radio and voice-over markets and has shot more than 200 television commercials.
As a screenwriter, he wrote Legendary, starring Patricia Clarkson and Danny Glover was released in 2010 where he also had a bit part in the film too. Another screenplay, Gorgeous George is currently in development.
He first came to the universe of Teen Wolf in the web series Teen Wolf: Search for a Cure where Scott and Stiles believe he may hold a cure for Scott werewolf problem.
He made a cameo appearance in Wolf's Bane and as of Season 4, his character began recurring in some episodes.