Craig Stadler, who owns two Champions Tour major titles, posted a 4-under 66 and is alone in third place at minus-9. David Edwards is one stroke further back at minus-8 after a 65 of his own.
Gil Morgan (65) and Tim Simpson (67) share fifth place at 7-under-par 133.
Romero carded two birdies on the opening nine holes to head to the back nine at minus-5. He birdied No. 11 and came right back to birdie 12 on the Ailsa Course at Westin Turnberry Resort.
The Argentine tripped to a bogey at the par-4 13th to slip back to minus-6. However, Romero bounced back with a birdie on 14.
The 52-year-old eagled the par-5 17th from four feet out to get to 9 under.
'I think that was the longest drive I have ever hit - it was 379 yards. Unbelievable!' Romero exclaimed. 'I had 110 yards to the hole and hit a sand-wedge to 4 feet and made the putt for eagle.'
Romero closed with a 3-foot birdie putt at the last to join Roberts on top of the leaderboard.
'I have been doing yoga now for five years, but I have decided to take a break this year to rest,' Romero stated. 'Now I can just focus on the putter because my big problem has always been my putter. For me, the putter was a terrible club. It was, but not now. I felt very, very strong today.'
Romero's best finish in a major while on the PGA TOUR was a tie for seventh place at the 1997 British Open Championship. Since turning 50 years old and becoming eligible to play on the Champions Tour, Romero tied for second at the 2004 Senior British Open and shared ninth at the same event last year.
Roberts notched back-to-back birdies from the par-4 second to move to 7 under. After a pair of pars, he birdied the par-3 sixth and also birdied seven.
The 2005 JELD-WEN Tradition winner birdie No. 10 for the second day in a row to move to minus-10. Roberts closed with eight consecutive pars to share the second-round lead.
Those eight pars to complete his round helped Roberts finish the opening 36 holes bogey-free.
Stadler, who also won the 1982 Masters, was even-par over his opening nine holes with two pars and two birdies. Around the turn, Stadler birdied three of the first four holes to jump to minus-8. A birdie at the last got him within one of the lead.
The cut line fell at 4-over-par 144 with 78 players advancing to the weekend. Two-time Senior British Open champion Bob Charles (145), Mark Johnson (145), Sam Torrance (148) and 2004 Senior British Open winner Pete Oakley (151) were among those missing the cut.
Defending champion Tom Watson is among those who will be around for the weekend. Watson shot 67 on Friday to move to even-par 140. Also making it to the weekend was three-time Senior British Open winner Gary Player, who made the cut on the number.