Montoya won his first Indy 500 with Ganassi, and then drove for Ganassi during his NASCAR stint from 2007 through 2013.
He spoiled Ganassi's 57th birthday on Sunday but said his former team owner congratulated him after the race. "We're still good friends," Montoya said.
Of his current seat with Penske, Montoya said, "I'm glad I am proving them right and that they made the right choice."
The win capped his remarkable comeback during the race. He started 15th, and a rear bumper panel on his car was knocked loose by the car of Simona de Silvestro on an early lap, forcing Montoya to the pits. The setback dropped him as low as 30th in the 33-car field.
But he steadily moved back through the field and, with less than 15 laps left, was swapping the lead with Power and Dixon until Montoya grabbed it for good.