Hallisay was born in Washington, D.C., graduated from Gonzaga College High School in 1996 and then moved on to Cornell University, graduating in 2000. After college and before moving to Los Angeles, he worked on Wall Street. He says, "I was slaving away at that and I had a really good time with it, but it got to the point where I said, "You know, I had always dreamed about seeing what acting was all about."

Hallisay made his television debut in 2005, appearing on Fox's The Inside, with Rachel Nichols and Adam Baldwin, as well as Lifetime's Strong Medicine, opposite Patricia Richardson.

He made his feature debut in Erik MacArthur's Bottoms Up, starring Paris Hilton and Jason Mewes. Hallisay played the boyfriend to Hilton, a socialite who helps Mewes save his failing small-town restaurant. In 2011, Hallisay was cast in the third installment of Hostel where he played the lead character. In 2012, Hallisay appeared in two episodes of The CW's series Ringer, and he signed a deal with Lifetime to appear as Kyle Parks in the drama series The Client List.

In March 2012, Hallisay began dating his The Client List co-star Jennifer Love Hewitt. In June 2013, it was announced that Hewitt and Hallisay were engaged and expecting their first child. They married sometime later in 2013, before the birth of their first child, a daughter named Autumn. In June 2015, the couple had a son named Atticus.