He was a teen idol during the late 1980s as David Witherspoon on the NBC family drama Our House and as Zach Nichols on the NBC sitcom My Two Dads[1][2] before transitioning to an adult career as Matthew Cooper on the CBS western drama Dr. Quinn: Medicine Woman.

[9] In 1983, he appeared on Cutter to Houston, playing "a kid who got hurt and had to be given mouth-to-mouth and carried to the waiting chopper by Dr. Hal Wexler (Alec Baldwin)".

On November 5 of that year, the three-part mini series Straight Up premiered, where he co-starred with Louis Gossett Jr.

In it Allen plays a teen tempted by drugs, while Gossett plays a magical character who operates the "fate elevator", so that each time Allen is tempted by a substance Gossett takes him on an elevator ride which shows the consequence of that substance.

Allen's next contract role was Matthew Cooper in Dr. Quinn, Medicine Woman alongside Jane Seymour (1993–98).

Starting with Third Man Out (2005), Allen stars as Donald Strachey, a gay private detective in a monogamous relationship, in a series of television movies for the here!

Allen noted that Strachey is the first gay character he had ever played outside of theater and that, though his career is "different" since coming out, he finds it "more interesting and fun for me than it has ever been.

[18] In 1996, at age 21, Allen was outed as gay when the US tabloid The Globe published photos of him kissing another man in a hot tub at a party.

[3] On January 17, 2006, Allen appeared on CNN's Larry King Live with San Francisco mayor Gavin Newsom to represent his viewpoint in a debate over same-sex marriage.

[6][21][22] In November 2006, The Los Angeles Daily News wrote in passing that Allen's partner, Jeremy Glazer, was also in the film Save Me.