The 16-year-old from New Zealand overcame a double bogey on her third hole with three birdies on the back nine at Tiburon Golf Club for a 1-under 71.
Ko was seven shots behind Sandra Gal of Germany, who opened with six straight birdies on her way to a 64.
Toronto's Rebecca Lee-Bentham had six birdies on the back nine for a 65. Another shot behind was a group that included Lexi Thompson, coming off a win last week in Mexico, and Anna Nordqvist, who can make more money by winning the CME LPGA Titleholders than she has all year.
First place is worth US$700,000, the biggest payoff in women's golf.
Ko wasn't thinking about money, even though this is the first event where she can take home a paycheque — and there is no cut in the 69-player field.
The starter introduced her by asking the gallery of about 100 people to "welcome to the LPGA Tour, Lydia Ko." She calmly hit driver down the left side of the fairway on the par-5 opening hole.
"Just normal," Ko said. "I didn't feel too odd or special or slow or whatever today. That actually surprised me. I thought I would be much more nervous. And actually, one of the good things was I wasn't thinking about any money or related stuff. I just tried to play my game, which was obviously very helpful."
She missed a five-foot birdie putt on the first hole, and then really ran into problems on the third. Ko pulled her tee shot through a waste area of coquina pebbles and just into the pine straw. She tried to play a draw toward the right side of the green, but was distracted when her club clipped a branch at the top of her swing, and her foot slipped. She didn't get out of the waste area, and then took two more shots to reach the green and made a four-footer for double bogey.
"I think I was a bit too ambitious," she said. "A seven-iron down the right side would have given me more than an opportunity to make up-and-down for par."
It probably wasn't a coincidence that Michelle Wie played in the same group. Wie also was 16 when she made her pro debut in the 2005 Samsung World Championship at Bighorn, a small field with no cut. She opened with a 70.
"On the first tee, watching her hit, I remembered my first shot," Wie said. "I remember shaking on the first tee. I hit the fairway and my hands went up in the air. There were definitely flashbacks to when I hit. She played great today. She looked calm — a lot calmer than I looked."
Ko can only hope for a better outcome.
Wie tied for fourth in her pro debut, but she didn't get the $53,126 because LPGA Tour officials determined she had taken a penalty drop in the wrong spot. She was disqualified for signing for the wrong score.
Ko did well not to let the round get away from her. She turned a birdie chance into a shocking bogey on No. 7 with a three-putt from just inside 15 feet, missing a two-footer for par when she tried to jam it into the back of the hole. That put her at 3-over through seven holes, as Gal was making birdie on the other side of the course on every hole.
"My birdie on 8 definitely helped," she said. "It kind of came in from the back of the hole, which was quite interesting. I thought I had missed it. I gave myself opportunities, and par is sometimes good. All I can do is just set up birdie putts, and then some will go in and some won't."
They were dropping for Gal. She shot to the top of the leaderboard, settled into a string of pars, and then had a big finish. Unlike the teenager, Gal was thinking about money, or at least the breakdown. The prize distribution from the $2 million purse is heavy at the top — $700,000 for first place, while second place pays just over $139,000 and third place is about $100,000.
"I thought this tournament was a little different than the other ones, so I thought, 'I'm just going to be really aggressive and just go for everything,' because really all you want to do is win here," Gal said. "Any other place it doesn't really matter that much. So that's kind of the mindset I had and I think that really helped me, and I just kept rolling in putts. So that was kind of fun."
The "Big 3" didn't have that much fun — Inbee Park, Suzann Pettersen and Stacy Lewis. They are the top three in the world and the LPGA Tour money list. Park, who already clinched player of the year, had a 68. Lewis made late birdies for a 71, while Pettersen opened with a 72. Pettersen and Lewis have to win to have any chance of claiming the money title.