After graduation, Carmack attended New York University to pursue an Arts degree at the Tisch School. He left NYU after he was discovered by a modeling scout and decided to have a go at the profession. His looks were exactly what Bruce Weber was seeking for Abercrombie & Fitch. The other catalogs and editorials that he has worked for include: Lord and Taylor, Macy's, Target, Who.A.U, Elle France, Nautica, Guess?, and Cosmogirl.

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 starred in the movie Lovewrecked with Amanda Bynes and Jonathan Bennett and decided to hone his acting skills in the theatres of New York and London.

From 16 March – 21 May 2006 Carmack starred in the Off Broadway production of Entertaining Mr. Sloane at the Laura Pels Theatre. The play, directed by Scott Ellis, also featured Tony Award nominees Alec Baldwin and Jan Maxwell.

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.

Carmack portrayed Susan Delfino's cousin Tim in the television show Desperate Housewives for one episode.

He portrayed Sam Reide, the main character of the film The Butterfly Effect 3: Revelations, the third film in The Butterfly Effect film franchise, released in theaters in January 2009.