Peel Regional Police were called to the scene at Cawthra Road and Eastgate Parkway shortly before 7:30 p.m. after a van and car collided.
The driver of the car was pronounced dead at the scene, and a female passenger in the same vehicle was taken to hospital. She has since been released.
The driver of the van attempted to flee on foot but an off-duty Peel police officer stepped in to direct two pedestrians to detain the suspect.
"Without their help it’s possible that this person could have gotten away," said Const. Thomas Ruttan, media relations officer with Peel police.
The officer waited with the driver of the car until firefighters arrived and then proceeded to arrest the other man.
"Definitely heads-up police work even when he's off-duty," Ruttan said.
Mihajlo Radovamovic, 32, of Mississauga is facing charges for failing to remain at scene causing death, excess blood alcohol, and impaired operation or excess blood alcohol causing death.
Police have not released the name of the deceased driver.