CROMWELL, June 20, 2013 (AFP) - Hoffman, who shared 45th at last week's US Open, eagled the par-4 10th and par-5 13th holes and after a birdie at the 15th needed to birdie the last three holes to match the US PGA's all-time record low round of 59.
He only managed a birdie at the 18th, but it was enough to put him atop the leaderboard by a stroke over Mahan, who shared fourth at the US Open, and two shots over 2012 Masters champion Bubba Watson.
Past major winners Zach Johnson and Webb Simpson were in a group sharing fourth on 65 that included fellow American John Merrick, Venezuela's Camilo Villegas, Canada's Graham DeLaet and Australia's Rod Pampling.
US Open winner Justin Rose of England fired a 67 to share 18th.
Defending champion Marc Leishman of Australia opened on 66. He finished one stroke ahead of Mahan and Hoffman last year to capture the crown at the TPC River Highlands for his first US PGA victory.
Hoffman's bogey-free round included birdies at the par-5 sixth and par-4 seventh, his pair of eagles sandwiched around a birdie at the 12th and birdies on two of the last four holes.
Mahan began on the back nine with a birdie, ran off three birdies in a row starting at the 13th and added another at 17, birdied the par-4 second and bagged two final birdies at the par-3 fifth and the sixth.
Watson opened with a pair of birdies but took a bogey at fourth and followed a birdie at six with a bogey on the next hole before closing the front nine with another birdie.
He added birdies at 11 and 13 before making an eagle at the par-4 15th and a birdie at 17.
Leading scores on Thursday after the first round of the $6.1 million US PGA Travelers Championship (par-70, USA unless noted):
61 - Charley Hoffman
62 - Hunter Mahan
63 - Bubba Watson
65 - John Merrick, Webb Simpson, Rod Pampling (AUS), Camilo Villegas (VEN), Graham DeLaet (CAN), Zach Johnson
66 - Chris Stroud, Tommy Gainey, John Huh, Padraig Harrington (IRL), Richard Lee, Patrick Reed, Marc Leishman (AUS), Chris Kirk
67 - John Rollins, Robert Streb, Dicky Pride, Bo Van Pelt, Lee Westwood (ENG), Jerry Kelly, Bryce Molder, Stewart Cink, Brendon de Jonge (ZIM), David Branshaw, Ricky Barnes, William McGirt, David Mathis, Justin Rose (ENG), Angel Cabrera (ARG), Nick O'Hern (AUS)