About 900 workers represented by Unifor Local 444 are on the picket lines at Integram Seating in Lakeshore.
Unifor and management at the plant failed to reach a tentative deal by the 11:59 p.m. Tuesday deadline.
The plant on Patillo Road makes seating for the Windsor Assembly Plant.
Originally, the strike deadline was 11:59 p.m. on Saturday, but late last week, it was extended.
Workers voted 99 per cent in favour of strike action.
The last strike at the plant was in 2016.