After Lehmkuhl's parents, a policeman and a nurse, divorced when he was five, his family moved to Norton, Massachusetts, and his mother remarried. Sometime after 2002, he changed his first name legally from Richard to Reichen.

Lehmkuhl had a cameo in an episode of Frasier in September 2003 as "Impossibly Handsome Man” followed by a cameo on “The Drew Carey Show.”

On July 2006, former 'N Sync band member Lance Bass told People Magazine that he is gay and in a "very stable relationship" with Lehmkuhl. The couple broke up in January 2007. Bass said they remained "good friends".

On May 1, 2007, the LGBT-interest television network here! announced that Lehmkuhl had joined the cast for the third season of its original gothic soap opera, Dante's Cove. He plays the role of Trevor, originally described as "a Business school graduate who comes to Dante's Cove looking to find himself."

LGBT-interest network Logo announced on June 3, 2010, that Lehmkuhl and boyfriend, model Rodiney Santiago had joined the cast of Logo's reality series, The A-List: New York. The low-rated series, frequently described as a "Real Housewives"-style show, was cancelled after two seasons. Since the airing, Lehmkuhl and Santiago are no longer a couple.

Lehmkuhl was working simultaneously as a physics Teacher at Crossroads School for the Arts and Sciences, FLIGHT instructor, and model in Los Angeles when he was approached by a casting Director for The Amazing Race. Lehmkuhl and Chip Arndt were a couple during the competition but have since split. Lehmkuhl moved to Dallas, Texas briefly after his win on The Amazing Race, but before all episodes had been broadcast. Reichen's spending habits at that time caused speculation that he had won The Amazing Race — and that he and Arndt had broken up. During the show, the couple was typically described as "Married" in the subtitles that are used to illustrate the relationship between team members (other teams being, for Example, "Best Friends" or "Father-Daughter").