The 30-year-old American fired a sparkling six-under-par 66 in mainly sunny conditions on the hosting Pebble Beach Golf Links, one of three venues being used for this week's PGA Tour event.
In pursuit of his sixth title on the US circuit, Mahan mixed seven birdies with a lone bogey to end the day level with little-known Briton Russell Knox, who carded a six-under 64 on the Shore Course at Monterey Peninsula Country Club.
South Korean Noh Seung-yul and Japan's Ryuji Imada were among a group of six players knotted at five under while defending champion Phil Mickelson opened with a one-under 69 at Monterey Peninsula.
Mahan, who finished second behind compatriot D.A. Points at the 2011 Pebble Beach National Pro-Am, was delighted with his start in near-perfect conditions.
"I hit a lot of good putts today and they were just kind of right on the edge or lipped out today," the Californian told Golf Channel. "I made a lot of good swings and had a lot of good opportunities.
"I am happy to get in with what I did. The weather is so beautiful and the course is very doable right now. It's nice to shoot a round when it's there for you."
Asked how he would tackle the Monterey Peninsula layout in Friday's second round, Mahan replied: "That's the challenge, just to adjust to the greens and maybe the way the courses are actually playing compared to here or to Monterey or Spyglass.
"It's a challenge but that's why we are pros. The great ones adjust and they go on to win this tournament."
Former world number one Lee Westwood, making his first PGA Tour start of the year, was also in a buoyant mood after firing a 68 on the hosting Pebble Beach Golf Links in the company of his father.
"There's a long way to go but I'm off to a nice start, a 68, and I don't think that does you any damage on any of these golf courses," the 39-year-old Englishman said after carding five birdies and one bogey.
Asked how much he enjoyed playing with his father on one of the game's most iconic layouts in the pro-am celebrity event, Westwood replied: "It's the biggest thrill. This is going to be one of the great weeks of the year for me."
Westwood's father, a former mathematics teacher who introduced his son to the game shortly before he turned 13, applauded his partner's form in the opening round.
"He played pretty well today," Westwood senior said before adding with a grin: "You can see where he got his swing from, though, when you look at me. He copies everything I do."
American left-hander Mickelson, who clinched his 41st PGA Tour title by four shots at last week's Phoenix Open, was disappointed with his putting after finishing with two bogeys in his last five holes.
"It's a lot more challenging to make a lot of putts up here than it is in Phoenix," the world number 10 said after totalling 30 putts. "Hopefully I'll get a good run tomorrow and try to shoot myself up into contention.
"One of the things I've learned over the years here is you need to be patient," added the four-times Pebble Beach National Pro-Am champion. "If I play just a good, solid 18 holes, I should hopefully be able to shoot something in the mid-60s."
Triple Major winner Padraig Harrington of Ireland opened with a level-par 72 at Spyglass Hill while US Open champion Webb Simpson returned a one-over 71 at Monterey Peninsula.
Twice Pebble Beach National Pro-Am champion Dustin Johnson made a poor start with a three-over 73 at Monterey Peninsula and former world number one David Duval fared even worse at Spyglass Hill, battling to a seven-over 79.