Wayne Levi, David Eger, Jose Maria Canizares and Mark McCumber finished tied for fourth at 5-under-par 66.
Ahern, who won the Canada Senior Open Championship during his rookie season on the Champions Tour in 1999, birdied two of his first three holes at Newport Beach Country Club. He added a pair of birdies starting at the par-4 sixth but stumbled at the par-3 eighth with a bogey.
The 54-year-old picked up steam on the inward nine. He birdied the 10th and the 12th to reach 5-under before another stretch of birdies. At the par-4 14th, Ahern ran home the first of two consecutive birdies to secure the outright lead after round one.
Hale Irwin, who cruised to a five-shot victory at this event last year, finished three shots off the pace alongside John Jacobs, Bruce Lietzke and Rafael Navarro at 4-under-par 67.
Andy Bean, who is making his debut on the Champions Tour this week, collected four birdies and four bogeys to finish in a group at even-par 71.