Dallas was born in Louisville, Kentucky. He met English actress Lara Pulver in 2003 while working at the Theatre Royal, Windsor. They married in 2007, and were divorced in 2011. After graduation, Dallas joined the Royal Shakespeare Company, and then took parts with the Royal National Theatre, English National Opera, the New Shakespeare Company and The Young Vic.
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 virtual 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 (Stan Lee, in 1965, created the character 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."
In 2011, he began starring on the ABC series Once Upon a Time as David Nolan/Prince Charming. Dallas did not reprise his role in the sequel to Thor due to his commitments in Once Upon a Time. Zachary Levi replaced him in the film.
Dallas began dating his Once Upon a Time co-star Ginnifer Goodwin in fall 2011. Dallas and Goodwin became engaged in October 2013, and married on April 12, 2014 in California. Together, Dallas and his wife have two sons: one born in 2014, the other born in 2016.
In May 2017, it was announced that Dallas, along with Goodwin, would not be returning as series regulars for the seventh season of Once Upon a Time. However, Goodwin revealed that both she and Dallas would return in whatever capacity they could as recurring guest stars.
Gallery of behind the scene stills released to promote the actor.