Although there has been much speculation surrounding Moennig's sexual orientation due to the on-screen lesbian and transgender roles she has taken, in a 2000 interview with Rebecca Traister of the New York Observer, Moennig identified herself as heterosexual. The only evidence to the contrary is a comment by L Word costar Jennifer Beals in an interview with The Advocate, which lumped Moennig with openly gay staff and cast members: when asked "Do people still assume that you have to be a lesbian to play one on TV?", Beals responded with, "Not at all... I heard a rumor that somebody... assumed I was bisexual. Which was a huge compliment to me, because often I will go to Leisha or Kate or Ilene or Rose Troche most often and ask, 'OK, is this the right thing to do? Is this not the right thing to do? Am I going to seem like a total chump if I do this?" Moennig has made no further comments beyond her initial identification as heterosexual.

Moennig is of German and Irish descent.

Career Moennig moved to New York City at the age of 18 to study at the American Academy of Dramatic Arts. There she began a modeling career, and also performed on stage.

In 1999, she had the central role of the Our Lady Peace video "Is Anybody Home?"

Her first major role was in the television series Young Americans, playing Jacqueline "Jake" Pratt, a girl who enters the Rawley Boys Academy by passing as a boy and ends up falling in love with Hamilton (Ian Somerhalder), the Dean's son.

She has played many lesbian and transgender roles. She auditioned for the part of Brandon Teena in Boys Don't Cry - that ended up going to actress Hilary Swank. She played a pre-operative transsexual in Law & Order: Special Victims Unit (episode "Fallacy").

Since 2004, Moennig has played one of the main characters in The L Word. She plays Shane McCutcheon, a hairdresser who goes through a succession of romantic partners.

On April 12, 2006, Moennig made her Off-Broadway debut in Guardians, by Peter Morris. In it, she plays "American Girl" - a young United States Army soldier from West Virginia who becomes a scapegoat in a scandal involving abuse at an Iraqi prison. The story is loosely based on that of Lynndie England.

Moennig had a minor role in the 2006 film Art School Confidential as Candace, the ex-girlfriend of Audrey, the art model who serves as a love interest for protagonist Jerome.

In 2008, Moennig played the role of Mary, a suspect in season 6 episode 19 of CSI:Miami.