Parrack was born on February 8, 1981 in Allen, Texas. He attended the co-ed Allen High School for secondary education. In 2001, Parrack moved to Los Angeles, California where he studied acting at The Stella Adler Academy and then at the Playhouse West.
Parrack made his screen debut in the 2006 drama film Annapolis. From 2006–2008, he made multiple guest appearances on television shows such as Monk, Grey's Anatomy, CSI: Crime Scene Investigation, and Criminal Minds. In 2008, Parrack was cast as Hoyt Fortenberry in the vampire television drama series True Blood. He was part of the main starring cast for the first five seasons, before departing for a season. Parrack returned to the show for the seventh and final season, again as a main cast member. In 2011, Parrack returned to feature films, starring in the military science fiction war film Battle: Los Angeles. The film was directed by Jonathan Liebesman and co-starred Michelle Rodriguez, Bridget Moynahan, Ne-Yo, and Michael Peña. In 2013, he starred in the drama film Child of God and the Spanish-American drama A Night in Old Mexico. He co-starred in the films Fury (2014), The Adderall Diaries (2015), and Suicide Squad (2016). In 2014, Parrack joined the Broadway cast of Of Mice and Men, playing the role of Slim.
Parrack married actress/writer/director Ciera Danielle on October 19, 2008. He and Danielle separated in September 2013 and in June 2014, it was reported the couple had filed for divorce due to irreconcilable differences. In Spring 2014, it was confirmed he was in a relationship with The Hunger Games actress Leven Rambin. On October 10, 2015 he and Rambin married in Texas. On May 9th 2017 the couple split ending their two-year marriage. Parrack is the president of 120 Productions, Inc. He is good friends with actor James Franco.