McKennie's close-range header on 78 minutes put Juve level after Nicolas Nkoulou had given Torino a first-half advantage.
After McKennie's equaliser, Leonardo Bonucci produced an 89th-minute winner to lift Juve to second in the table, three points behind AC Milan who visit Sampdoria on Sunday.
Inter Milan can overtake Juve by beating Bologna in Saturday's late match.
Before the game in Turin, Cristiano Ronaldo was presented with a special jersey commemorating his 750th career goal.
But Ronaldo played a supporting role for once as 22-year-old McKennie, on loan from German club Schalke 04, scored a first Serie A goal and was mobbed by his teammates.
Juve travel to Barcelona in their final Champions League group game on Tuesday, but both clubs are already assured of place in the last 16.
December 5, 2020