Carmack was born in Washington, D.C., and grew up in Derwood, Maryland. He has two siblings, a brother and sister. As a young boy, he enjoyed participating in all types of sports: baseball, basketball, football, and wrestling. Carmack was also a longtime counselor at Seneca Creek Day Camp, known for singing the original "Rocko the Lazy Moth" and "Ode to Salpino" among other favorites. At Magruder High School, however, his attentions turned to theatre and he began working on three productions per year, on-stage and off, in addition to competing in numerous drama festivals.
After two years in New York City, Carmack decided to move to Los Angeles and immerse himself in acting. He is best known by global audiences as Luke on the first season of The O.C., but did not continue because producers could not guarantee him airtime in the upcoming year. In 2005, Carmack became a model for Ezra Fitch, a limited upscale label sold at Abercrombie & Fitch.
In 2006 Carmack made his London stage debut in a rare revival of Tennessee Williams' Summer and Smoke, a play unperformed in London for 55 years. Carmack starred opposite Rosamund Pike and was directed by former Royal Shakespeare Company artistic director Adrian Noble. It played the Apollo Theatre from 21 October–25 November 2006, failing to complete its scheduled run.
In 2013 he was cast in the ABC drama series Nashville as Scarlett and Gunnar's neighbor Will, a sexy and charming country singer and guitar player. Carmack's character initially appeared to be a womanizer and a potential threat to Scarlett and Gunnar's relationship, but it was revealed that the character is gay, and trying to protect his image as a heterosexual country music heartthrob. Carmack appeared in the final six episodes of the season, and was promoted from a recurring role to a series regular for season 2.