A second thug pleaded guilty yesterday in a mugging spree that resulted in the death of an elderly nun.

William Robbins, 20, wasn’t driving the car that hit Sister Mary Celine Graham, 84, at West 122nd Street and Lenox Avenue two years ago, but was just as liable, prosecutors argued.

Robbins will get 15 years in prison. He and Dyson Williams were screeching away from a mugging when cops stopped their car. Robbins got out, but Williams sped away, hitting the nun.

Williams already pleaded guilty, and will serve 18 years.