With winds making things difficult at the La Vallee du Richelieu Golf Club's Rouville Course, Mast shot a 3-under-par 69 for a one-shot advantage over Kenny Perry.
Perry is trying to add to his 789-point lead over Langer in the yearlong race for the Charles Schwab Cup.
Mast played early in the morning before the worst of the winds began buffeting the course.
"I think we got a little bit of a break playing second off," Mast said. "It was windy, but not as windy as it got later. We had a chance to play four or five holes without real heavy wind and that's an advantage."
Mast's best finish on the over-50 circuit this year has been a tie for 15th at the recent Boeing Classic in Seattle.
Defending champion Mark Calcavecchia, having missed the tour's previous two events with back problems, shot a 74.