Marecic was recruited to Army, Yale, and Stanford, eventually opting to attend Stanford. In his first three seasons with the Cardinal, Marecic was used primarily as a fullback, blocking for Toby Gerhart. He was an excellent blocking fullback, and seldom carried the ball. In his junior year Marecic was brought in as an inside linebacker in short yardage situations. In his senior season of 2010, Marecic’s part-time linebacker assignment became full-time, a decision the coaching staff made reluctantly out of necessity. They later praised his conditioning for the challenge of participating in so many full-contact plays–including over 100 in a single game–through yoga, stretching, meditation before switching positions, and a thorough academic understanding of the difficulties as a human biology major. He was the only player in the Football Bowl Subdivision to start on both offense (at fullback) and defense (at inside linebacker). In the fourth game of the season against Notre Dame, Marecic scored a pair of touchdowns within 13 seconds of one another, first as a fullback on a short dive play, then 13 seconds later making an interception from his inside linebacker position, running it back for a score. In doing so he became the first player to score touchdowns on both offense and defense in the same game since Eric Weddle in 2006.

On January 10, 2011, he was named the inaugural winner of the Paul Hornung Award as the most versatile player in college football. Marecic also finished in 10th place for the 2010 Heisman Trophy, receiving 3 first place votes.

On April 30, 2011 Marecic was selected in the 4th round by the Cleveland Browns with the 124th pick in the 2011 NFL Draft. This was one of four picks the Browns acquired by trading their own first round pick to the Atlanta Falcons; the Falcons drafted future pro-bowler Julio Jones, while the Browns drafted Phil Taylor (DT), Greg Little (WR), Marecic, and Brandon Weeden (QB) and have since been widely criticized. He was projected to play at fullback rather than linebacker. The Browns were reportedly impressed with his work ethic and approach to the game.

Marecic made the Browns roster, contributing in 2011 primarily as a blocking back for running backs Peyton Hillis and Montario Hardesty. He was cut by the Browns on August 27, 2013.

On September 17, 2013, the 49ers signed Marecic to a one-year contract. Marecic had played at Stanford under then-49ers coach Jim Harbaugh. He was released on October 1, 2013 without playing in an NFL game. According to the 49ers, Marecic decided not to continue playing in the NFL.