Pasdar was born in Pittsfield, Massachusetts. His mother, Rosemarie (née Sberesny), was born in Königsberg, Germany, and worked as a nurse before becoming an English teacher and travel agent in France. His father, Homayoon Pasdar, was a cardiac surgeon who was born in Iran and moved to the United States, working as a surgeon near Philadelphia.

At the age of 19, he auditioned for a role in Top Gun. Director Tony Scott was so impressed that he wrote the part of "Chipper" just for him. This led to bigger roles in Solarbabies (1986), Streets of Gold (1986), and Kathryn Bigelow's cult vampire movie Near Dark (1987), with Pasdar in the lead role of Caleb Colton. Other major roles include Vital Signs (1990). Pasdar got his biggest break in movies, when he starred as a beautiful woman opposite Julie Walters in the British movie Just Like a Woman. In 1992, he left Hollywood and returned to New York, working as a cashier for room and board, while taking the occasional small part, such as Frankie in Brian De Palma's Carlito's Way (1993).

Pasdar's major break into television came in 1996, when he was cast as the title character on the short-lived Fox series Profit. From 2000 to 2002, Pasdar played the lead role of Anthropology Professor Declan Dunn in the spooky cult drama series Mysterious Ways on PAX. Pasdar played David McClaren in the final two seasons of the long-running CBS drama Judging Amy, from 2003 through 2005. In 2006, he had a high-profile guest role as Gabrielle Solis's sleazy lawyer in Desperate Housewives. He currently stars in the NBC superhero drama Heroes as Nathan Petrelli. He bases his mysterious character on "the most morally liquid characters" that he's encountered in his life. The character is not based on one particular political figure, but on a melange of different ones, both good and bad.

Pasdar married lead singer Natalie Mainesof the Dixie Chicks in June 2000. The couple first met in May 1999, with Maines as a bridesmaid to her bandmate Emily Robison and Pasdar as a groomsman to his friend singer-songwriter Charlie Robison. They were married in Las Vegas's Little White Wedding Chapel, in a $55 no-frills ceremony officiated by the chapel's "Pastor Ann." They have two children, Jackson Slade Pasdar (born March 15, 2001) and Beckett Finn Pasdar (born July 14, 2004) and live in Los Angeles, California and Austin, Texas. He has a tattoo of an anchor on his arm. His children's names are written on it. It symbolizes that his family is his anchor and support in life. He also has a tattoo of the Chinese character for Strength, which he got while filming Shanghai 1920 .