Lehman's six-under-par 66 left him a stroke ahead of fellow Americans Paul Goydos and Jim Furyk, while at three-under were Fred Couples, Guy Boros and Justin Leonard. Tiger Woods led a group of eight players on 70.
The Mercedes is the opening event in the USPGA Tour's season, with the 32 winners of 1996 tournaments making up the field.
Lehman is playing as though there was no two-month gap since the last official event, the Tour Championship, which he won by six strokes. "I started slow and didn't feel really focused," he said, "but I made a bogey on the fifth hole and that got me back into the competitive mode and I made seven birdies in 13 holes."
"The key to the round was that I didn't expect too much of myself. I didn't practice that much during the off season. There was no pressure to play well, no huge expectations but once you start making a few putts the hole starts looking bigger and it all gets a bit easier."