The 400 workers, members of the Canadian Auto Workers union, set up picket lines at midnight after contract talks collapsed late Saturday night.
The two sides remain far apart over what the union says are Lear's demands for deep concessions at a time when the company is making significant profits.
Last week the workers voted 97 per cent in favour of a strike to back their demands.
Lear Whitby manufactures seats for vehicles produced at General Motors in Oshawa and there were fears a strike could have a major impact on GM operations in the city.
The Lear strike comes on the heels of another strike at auto parts company Wescast, in Strathroy Ontario which began Saturday morning.
The Wescast strike resulted from a decision by General Motors which would see the current work performed at the facility go to China.
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