Carmack grew up in Rockville, Maryland. As a young boy, he enjoyed participating in all types of sport: baseball, basketball, football, and wrestling. At Magruder High School, however, his attentions turned to theatre and he began working passionately on three productions per year, on-stage and off, in addition to competing in numerous drama festivals.
After graduation, Carmack attended New York University to pursue a degree in Arts in the Tisch School, but left after his sophomore year. He was discovered by a modelling scout and decided to have a go at the profession. His looks were exactly what Bruce Weber was seeking for Abercrombie & Fitch. The other catalogues and editorials that Carmack has appeared in include: Lord and Taylor, Macy's, Target, Who A U, ELLE France, Nautica, Guess?, and Cosmo Girl.
After two years in New York City, Carmack decided to move to Los Angeles and immerse himself in acting, and is well known by global audience for his appearance as Luke on the first season of The O.C.. Carmack also had an uncredited guest role on a 2000 episode of Dawson's Creek. He is currently in a production of Entertaining Mr. Sloane, which opened on March 16, 2006 Off-Broadway at the Laura Pels Theatre in New York. The show also stars Alec Baldwin and Jan Maxwell.
As of 2005, Carmack is a model for Ezra Fitch, the newest offspring brand from Abercrombie & Fitch.
He currently stars on Nashville, playing gay country singer, Will Lexington