Bruce Fleisher was part of a three-way tie for fourth at five-under-par 67. He finished alongside John Jacobs and Mike McCullough.
Davis was the first of the leaders to reach the clubhouse thanks to an excellent back nine at Hayfields Country Club. After picking up three birdies and one bogey on the front side, the Australian torched the inward side for a share of the lead.
At the 10th, Davis landed a pitching wedge inside two feet for birdie and placed the ball within three feet at the following hole to make it two in a row.
The 51-year-old birdied the 13th and added another at the par-four 15th to reach six-under. At the very next hole, a par-five, Davis hit his third shot onto the putting surface and drained the 10-footer to hold a share of the lead after the first round for the first time in his career.
'I played well from tee to green,' said Davis, a long-time fixture on the European and Australia tours. 'Tee to green is one of my strengths, that's why you see me with [the long putter] in my hand.'
Ahern, who won the 1999 Canada Senior Open Championship, cruised to a bogey- free round for his share of first.
At the par-four 13th, Ahern rolled in a 50-foot putt for birdie to move to six-under. He then two-putted for birdie at the 16th to join Davis.
'It was perfect when I started this morning,' said Ahern. 'Wind was a non- factor most of the day. It was ideal conditions today.'
Mahaffey, a 10-time winner on the PGA Tour, also played a flawless round to join the logjam atop the leaderboard.
The 54-year-old, who won the 1978 PGA Championship at Oakmont, made a 30-foot birdie at the 13th and drained a 15-footer at the last to finish at minus- seven.
'This course is in fabulous shape,' said Mahaffey. 'I haven't played as well out here as I could have. The competition is better than people think. There are a lot of good players out here.'
Fleisher, who has finished second at this event twice, had six birdies and one bogey to finish two shots behind the leaders.
Bobby Walzel, Bobby Wadkins, Doug Tewell and Dale Douglass were tied for seventh at four-under-par 68.
Defending champion Allen Doyle collected two birdies on his round for a two- under-par 70. Doyle, who is seeking his first victory of the season, finished tied for 20th, five shots off the pace.
Full field scores from the Greater Baltimore Classic