Masingill stands at 1-under-par 359. He was joined in the lead by Japan's Massy Kuramoto, who posted a 2-under 70 in windy conditions on the Champions Course at PGA Southern California Golf Club.
After the Santa Ana winds calmed on Saturday, they were back up on Sunday. Just five players carded rounds under par on Sunday, while only two more shot even-par 72.
Bill Longmuir shared fifth place entering round five, but managed one of the two rounds of 72 to inch up to third place at even-par 360.
Fourth-round leader Kirk Hanefeld stumbled to a 4-over 76 and is one stroke back at plus-1. He was joined there by second- and third-round leader Joe Ozaki (73). Former major league baseball pitcher Rick Rhoden is alone in sixth at 2-over-par 362 after a 3-over 75 Sunday.
After Monday's sixth and final round, the top-seven players gain full-exempt status on the Champions Tour, while the next eight players earn conditional status.
Darrell Kestner, whose best finish on the Champions Tour came earlier this year at the Long Island Classic where he tied for sixth, fell back into a tie for seventh at 3-over-par 363 after a Sunday 74. He was tied there by Jack Ferenz, who posted a 70 in round five.
Masingill opened with a birdie on No. 1 to move to 3 under. He bogeyed the third, but came right back with a birdie on four.
He faltered back to even-par for the tournament with bogeys on six, eight and 10. Masingill finally righted the ship with a birdie on 11. However, he gave that stroke back with a bogey at the next. He picked up a birdie on 16 to share the overnight lead.
Kuramoto posted two birdies on the front nine to climb to minus-1. He dropped a stroke at the par-4 10th before converting a birdie on No. 12. Kuramoto moved to 2 under with a birdie on the par-5 16th, but he bogeyed the last to end at minus-1.
'It was difficult playing today with the wind. The wind was probably the worst we have played in so far,' said Kuramoto, a long-time star on the Japan Golf Tour. 'My strategy was to keep the ball low and try to place it on the green so I was putting into the wind.'
Eduardo Romero, a European Tour veteran, carded a 1-under 71 to end at 4-over-par 364. He was joined in ninth place by Keith Fergus (74) and Rick Karbowski (75). Norm Jarvis struggled badly on Sunday with a 6-over 78 that dropped him to plus-5.