After high school, Mahan enrolled at the University of Southern California, where he was named Pacific-10 Conference Freshman of the Year.

Mahan turned professional in 2003 and made it through qualifying school to earn a PGA Tour card for the 2004 season.

His first PGA Tour victory, which came at the 2007 Travelers Championship, lifted Mahan into the top 100 of the Official World Golf Rankings.

On February 28, 2010, Mahan won his second PGA Tour event, beating fellow OSU golfer Rickie Fowler by one stroke at the Waste Management Phoenix Open.

[6] Mahan recorded his fifth career PGA Tour victory in April at the Shell Houston Open and moved to fourth in the Official World Golf Ranking, making him the highest-ranked American for the first time.

[7] In defense of his title he won in 2012, Mahan reached the final of the 2013 WGC-Accenture Match Play Championship but was beaten 2&1 by Matt Kuchar.

He won by two strokes from Stuart Appleby, Jason Day and Cameron Tringale during a final round that saw six different players share the lead at some point.

[8] Mahan was selected by Tom Watson as one of his three captain's picks for the 2014 Ryder Cup team, finishing with a record of 1–2–1 in the four matches he participated in.

QF, R16, R32, R64 = Round in which player lost in match play "T" = tied Note that the HSBC Champions did not become a WGC event until 2009.