Haas was a member of the Nationwide Tour in 2005 after failing to earn his PGA Tour card in Qualifying School. His best finish in a Nationwide Tour event was a solo 2nd at the 2005 Scholarship America Showdown.

Haas birdied the last two holes at the 2005 qualifying tournament to earn his card on the PGA Tour for the 2006 season where he posted six top-25s. In 2009 Haas shot his lowest ever round on the PGA Tour with an 8-under 62 to take the first-round lead by two strokes over Brandt Snedeker, Chez Reavie and Ryan Moore at the Valero Texas Open.

In 2010 he won his first PGA TOUR title at the Bob Hope Classic. He two-putted for birdie in a Monday finish to shoot a final-round, 8-under-par 64 and beat Matt Kuchar, Tim Clark and Bubba Watson by one stroke.

In 2011 Bill抯 first win of the season turned out to be the most significant of his career. Winning the Tour Championship and with it the FedEx Cup, these twin triumphs saw him collect a staggering total of $11.44 million - $1.44 million for the tournament triumph and an extra $10 million bonus for the FedEx Cup. A further win for Haas in 2011 came in the form of the Presidents Cup under Fred Couples captaincy, to which Haas contributed 1 and a half points to the winning team.

In 2012 Haas won for the fourth time on the PGA Tour at the Northern Trust Open. He came from two strokes back on the final day to hold the clubhouse lead, resulting in a playoff with Phil Mickelson and Keegan Bradley. Haas then holed a 43 footer for birdie ensuring his victory.