Jacobsen, who has won two Champions Tour majors, was joined atop the leaderboard by 2005 JELD-WEN Tradition winner Loren Roberts and former Masters champion Craig Stadler.
Des Smyth, who lost to Tom Watson in a playoff at this event last year, shares fourth place with Tim Simpson at minus-4 on the Ailsa Course at Westin Turnberry Resort.
Jacobsen, who is battling a bad hip, tripped to a bogey on the first. He ran off four straight birdies from the par-3 fifth to get to 3 under.
After a pair of pars, Jacobsen birdied the par-3 11th. He moved to 5 under with a birdie on the par-4 14th, then gave that stroke back with a bogey on the 16th. Jacobsen came right back with a birdie on 17 to end at 5 under.
'I have been struggling with a bad hip, which is going to need some medical attention this winter and I wasn't sure I was even coming,' Jacobsen admitted. 'But because of this venue and the fact I love playing in the Seniors Open, I did everything I could to come.'
The bad hip nearly caused Jacobsen to stay home, but he decided to fly over after hitting balls last Sunday.
'I had a couple of procedures last Thursday. I had an epidural to reduce some pain in my back and also some platelet gel injected into my left hip,' stated Jacobsen. 'My doctor told me to wait until Sunday morning to hit balls to see how I felt. So I went out and hit some balls and jumped on a plane at two o'clock on Sunday afternoon.'
Roberts was steady throughout his round. He birdied the par-3 fourth to get on the board. After pars on the next four holes, Roberts birdied the ninth to turn at minus-2.
The 51-year-old Roberts came right back with a birdie at the 10th. He got within one of the lead thanks to a birdie at 13. A birdie at the par-5 17th gave Roberts a piece of the first-round lead and helped close out a bogey-free round.
'I didn't really scare a bogey today and that is very unusual for links golf,' Roberts said. 'The weather conditions were great today, more like California, and it was there for the taking. Peter put up a good score in the morning and me and the afternoon boys probably even had better conditions.'
Stadler, a two-time major winner on the Champions Tour, birdied the first, but quickly gave that stroke back with a bogey on No. 2. He drained consecutive birdies from the third to get to minus-2.
The 53-year-old made the turn at 3 under par thanks to a birdie at nine. After pars on four straight holes, Stadler birdied the 14th. He tripped to a bogey on 15, but eagled the par-5 17th to get to 5 under.
There is a logjam of players in sixth place. Former U.S. Open champion Scott Simpson carded a 3-under-par 67 and was joined there by Joe Ozaki, Gordon Brand, Juan Quiros, Eduardo Romero, John Ross, Scott Simpson, Mark James and the Edwards brothers, Danny and David.
Watson, the defending champion, posted a 3-over-par 73 and stands in a tie tied for 82nd.