Mahan, 31, was warming up on the range when he got a call informing him of his wife's condition. He planned to fly from Toronto to Dallas to be with her.
Mahan's withdrawal left John Merrick, who trailed Mahan by two shots after 36 holes and was to be paired with him in the third round, to play by himself on Saturday.
Kandi Mahan was reportedly not due to give birth until the week after the PGA Championship.
Mahan had been scheduled to tee off at 1:30 p.m. ET, but an 80-minute rain delay pushed his tee time back to 2:50 p.m. He never made it onto the course.
Later, Mahan issued the following statement:
“I received exciting news a short time ago that my wife Kandi has gone into labor with our first child. As a result, I have withdrawn from the RBC Canadian Open to return to Dallas. I would like to extend my very sincere gratitude and appreciation to RBC and the RBC Canadian Open.
'Kandi and I are thrilled about this addition to the Mahan family and we look forward to returning to the RBC Canadian Open in the coming years.”
Kandi Mahan is expecting a girl, whom the couple will name Zoe.