Jacobsen, who turned 50 last week, was joined by Craig Stadler, Don Pooley, Gil Morgan and Mike McCullough in a tie for first.
Defending champion Tom Purtzer was one shot further back at 4-under-par 68 along with Larry Nelson, Jose Maria Canizares and Mark James.
Jacobsen wasted little time in making his mark at Valencia Country Club and reached the green in two at the par-5 first. He two-putted for an opening birdie and played his approach to 20 feet for a birdie at the second to make it two in a row.
'I was so excited to play today,' said Jacobsen. 'It's important to get off to a quick start no matter where you are playing, but it's different here because it's only three days.'
At the par-4 fifth, Jacobsen dropped his second shot inside six feet and ran home the putt for his third birdie of the day.
He stumbled with a bogey at the par-3 seventh but recovered with a birdie at the ninth to make the turn at 3 under.
Jacobsen continued his fine play on the inward half and hit a brilliant approach shot within a foot of the hole for a tap-in birdie at the par-4 11th.
He faltered with a bogey at the 15th but made a late push for his share of the lead.
Jacobsen knocked his second shot to six feet for a birdie at the par-4 17th and converted an eight-foot birdie putt at the par-5 last to complete an impressive first day's work on the senior circuit.
'I made my share of putts, but there were a few more that I could have made,' said Jacobsen. 'That's golf.'
Stadler, who made a strong Champions Tour debut of his own in 2003, did most of his damage on the front side with four birdies over his first nine holes.
The four-time Champions Tour winner added a birdie at the par-4 10th and parred his way in to join the logjam in first.
Tom Kite, who won this event in 2002, finished two shots off the lead after a round of 69. He was joined by Hale Irwin, Allen Doyle, John Jacobs, Des Smyth and Raymond Floyd in a tie for 10th.
Wayne Levi, Doug Tewell, David Eger, Jim Colbert and Hugh Baiocchi were one shot further back at 2-under-par 70.