Dave Eichelberger, who almost withdrew less than 24 hours earlier because of bad back, is alone in second place after shooting a delicate 66.
John Bland is in third place at 5-under. Jay Sigel, who won last week's Farmers Charity Classic, is tied for fourth with Dave Barr at minus 4.
Mason had eight birdies and one bogey in posting his career low round on the Senior Tour. His previous best score came in the final round last week.
'Last week, I shot 68 in the last round and it really could have been a 65 after I shot 78 the previous day,' said Mason, a teaching pro from Dillard, Ga. 'I finally I just said: 'I've got to just let myself play golf.''
Mason shot 67 Monday at the Green Brook Country Club in nearby North Caldwell to grab one of the four spots left in the field of 78. He then paired with Mike Parish, who has caddied for the past 20 years at the Upper Montclair Country Club.
Only seven players in tour history have won a tournament after qualifying that week. Jim Ahern was the last, beating Hale Irwin in a playoff to win the 1999 AT&T Canada Senior Open.
Mason's 65 was a pleasant surprise, so, too, was Eichelberger's 66.
After driving to the course Thursday from his home in New Canaan, Conn., Eichelberger had trouble getting out of his car because his back hurt so much. He was excused from the pro-am and was treated by a physical therapist twice.
'I absolutely couldn't get down low enough to hit it yesterday,' said Eichelberger, who won the rain-shorted Emerald Coast Classic in March. 'I was just trying to get in position to hit the ball today, and then after three holes it loosened up.'
Eichelberger had eight birdies and two birdies in round one.
'It was a 100-to-1 I would be sitting here before I teed off,' Eichelberger said after his round.
Former kicker Al Del Greco, a five-time winner, and former defensive back Dick Anderson shot 73s to lead 12 current and former players into the fourth round of the NFL players portion of the tournament.
Full-field scores from the NFL Senior Classic