Lowly Peterborough salvaged a point at the KC Stadium as Hull's eight-match winning run in the league came to a halt.
The Tigersl took the lead in the second half with goals from Nick Barmby and Stuart Green, after Steve Thomson's opener for the visitors - but Willock tapped home in added on time.
Hull started slowly and it was no surprise when Willock set up Thomson to fire home the opening goal after 25 minutes.
However, Willock missed a glorious opportunity to score a second when he out-paced Damien Delaney, but was denied by goalkeeper Boaz Myhill.
Less than 30 seconds into the second half Hull drew level when Mark Tyler dropped Marc Joseph's cross from the right and Barmby headed home.
Fifteen minutes later Ian Ashbee released Green, who lifted a terrific finish over Tyler.
However Peterborough never stopped running and Willock was in the right place to convert Andy Clarke's cross from the left in stoppage-time.