As a boy, he played the autistic character "Tommy Westfall" on the TV series "St. Elsewhere" (1982). At first a minor character, Tommy took on great significance in the final episode when the entire series was revealed to have taken place inside his head. This has given rise to a fan theory known as the "Tommy Westphall Universe Hypothesis", which states that numerous shows that are connected to "St. Elsewhere" through character crossovers or dialog references have also taken place in Tommy's mind. For instance, the show had a crossover with "Cheers" (1982), and several characters from "St. Elsewhere" were featured in "Homicide: Life on the Street" (1993). From those links, the program can be linked to at least 280 other non-animated shows, ranging from "I Love Lucy" (1951) to "CSI: NY" (2004) (as of 2007).
Only signed on for the "Dr. Quinn, Medicine Woman" (1993) pilot to earn money for college. He did not think the series would be picked up and thought that it would pay for his college tuition. The series ran for 6 years.
One of the co-founders of The Creative Outlet theater company in 1995, and has appeared in several theater productions during the summer breaks.
One of the co-founders of a production company called Mythgarden, which he formed with producer Christopher Racster and actor Robert Gant ("Queer as Folk" (2000)).
Has been featured on the cover of "The Advocate" three times: October 9, 2001, November 25, 2003, and August 30, 2005.
Travels to Costa Rica are featured in the February 2004 issue of "The Out Traveler" magazine.
As of 2006, he has a position on the Honorary Board of Directors for The Matthew Shepard Foundation. On July 22, 2006, he joined Shepard's mother Judy and about 800 other people in a peaceful demonstration outside the Colorado Springs, Colorado, headquarters of Focus on the Family, a Christian organization.
Allen has been with his partner, actor Jeremy Glazer, since May 2005. They both appear in the film "Save Me" (2007).