Hasay, a Mission Prep graduate, came in with a time of 2 hours, 25 minutes and 20 seconds, the organization said.
Running his first Boston Marathon, Lawrence Cherono of Kenya won the men's competition in the closest finish in the men's race since 1988.
Krista DuChene finished her third Boston Marathon with a time of 2:44:12, eight seconds faster than her time in 2018.
Ethiopia's Worknesh Degefa won the women's elite race by 42 seconds with a time of 2:23:31, holding off Kenya's Edna Kiplagat (2:24:13), Jordan Hasay of the United States (2:25:20), Meskerem Assefa of Ethiopia (2:25:40) and Des Linden of the USA (2:27:00).
Cherono, Desisa and Kipkemoi made a move and remained locked together until the closing stages, with Cherono and Desisa kicking away and sprinting for the finish line.
Registration for the 2020 Boston Marathon will likely open in September, according to the Boston Athletic Association (B.A.A.). McCormack finished in 11th with a time of 2:30.38.