Dallas graduated in 1997, at the age of 15, from New Albany High School in New Albany, Indiana, where he studied theater under the wing of David Longest. Aged 16, he gained the "Sarah Exley Scholarship", that fully funded his degree level studies at the Mountview Conservatoire for the Performing Arts in London, England.
Returning to the United States, and based in Hollywood, he was cast in 2011 as Fandral in Thor, after Irish actor Stuart Townsend withdrew from the role days before filming was to begin. As he was a virtually unknown actor when cast, there was some speculation that Dallas was selected because there simply was not enough time to find a bigger name replacement. To prepare for the role, Dallas viewed Errol Flynn as an inspiration (the character was also created based on Flynn), and watched many of his films. He commented "I tried to bring out that little bit of Flynn-ness in it. Flynn had a lot of that boyish charm and Fandral's got all that in him."
Dallas met British actress Lara Pulver in 2003, when he came to the UK on a theater training scholarship. They married over Christmas 2007, in a 16th century barn in Devon. The couple honeymooned in the Maldives. Dallas confirmed the couple's divorce in an interview on the Bob Rivers radio show on December 2, 2011. He has been dating co-star Ginnifer Goodwin - who plays his leading romantic interest, Snow White/Mary Margaret, on Once Upon a Time - since fall 2011. On October 8, 2013, the couple announced their engagement and later married on April 12, 2014. Their first child, a boy named Oliver Finlay Dallas, was later born on May 29, 2014. On November 17, 2015 it was announced that Goodwin and Dallas were expecting a second child.