Recently crowned Celtic Manor Wales Open champion McDowell had started the day on level par at Pebble Beach Golf Links but reached the turn at two under thanks to an eventful four-birdie, two-bogey opening nine holes from the 10th tee.
At that point the 30-year-old was a shot behind first-round co-leader Brendon De Jonge of Zimbabwe, who had jumped out into the outright lead after beginning the day in a tie at two under par with England's Paul Casey and American Shaun Micheel following opening 69s.
Both McDowell and De Jonge had started their rounds at the 10th tee with the Zimbabwean at four under after six holes before recording three bogeys in the next four holes to find himself at one under having played 11.
McDowell, meanwhile, was striking out on his own, and having gone out in 34, two under for the day, he added more birdies at the par-four fourth and par-five sixth.
With three holes to play the Portrush man held a three-shot lead at four under par over De Jonge, American Dustin Johnson, who had played 12 holes, and Germany's Alex Cejka, after 15, with the afternoon wave of players set to start their second rounds.
Of the other overnight co-leaders, 2003 US PGA champion Micheel had birdied his second, the par-four 11th, to reach three under and was the first to show on a much cooler, cloudier morning than the previous day on the Monterey Peninsula when the average score was 75.288.
The American did not build on his early success, however, with four bogeys, including three in a row after his turn, at the first, second and third, to fall to one over par. World No 9 Casey had made a steady start with pars at his first four holes, only to card a triple-bogey eight at the 14th.
Further down the leaderboard, Rory McIlroy and Sergio Garcia appeared to be fighting a losing battle as both slumped to nine over par, each having played 13 holes.
Malton's Simon Dyson looked unlikely to make it through to the final two rounds after he followed up his opening round of 76 with a 74, leaving him eight over.