Mason managed a par 72 and was at 8-under 134 after two trips around the TPC of the Twin Cities. Irwin carded a 2-under 70, erasing a two-stroke deficit and moving into position for his third win of the season.
A club pro from Dillard, Ga., Mason stunned the field Friday by setting the course record by posting his lowest round on the elder circuit.
The leading money winner in Senior Tour history, Irwin also dropped off a bit after opening with a 66. He has 34 career titles to one for Mason, who won the NFL Golf Classic in June as a Monday qualifier.
The men played together Saturday and will do so again Sunday. They were at 8-under 136, three shots in front of a group of five that included Bob Gilder, who won a pair of tournaments in playoffs last month.