McKinnon tore his ACL a week before the start of the 2018 season, then was hurt again in training camp the following year. Rather than release him in 2020, San Francisco altered his contract to cut his basic pay and delete the last year, allowing him to become an unrestricted free agency in the 2021 offseason. He decided to retire instead. During his time with the 49ers, McKinnon started 31 of 32 games he appeared in and recorded 1,272 yards and seven touchdowns on 250 carries (5.0 avg.) while also catching 38 passes for 393 yards and two more scores. He announced his retirement on March 24, 2020. The 49ers released a statement saying they "regret losing Jerick" and that they hope to see him back at some point in the future. McKinnon responded by writing "It's been an honor to wear the green & gold". After going unselected in the 2019 NFL Draft, McKinnon signed with the New York Giants as a free agent on May 20, 2019. He was released by the Giants during final roster cuts on September 1, 2019. In 2016, McKinnon led the 49ers in rushing with 997 yards and eight touchdowns on 232 attempts (4.7 avg.). He also caught 29 passes for 217 yards and another score.