He is responsible for dropping the brick on five year old Odetta's head which caused her to split into two personalities. He is also responsible for pushing Odetta in front of the train that severed her legs. Jack also pushed Jake in front of the Cadilliac which killed him.
When Roland enters his mind through The Pusher door and takes over his body, he forces Mort to buy bullets for his guns and antibiotics. He proceeds to take him to the subway station while being pursued by the police. Roland kills Mort by making him thow himself beneath the train - the same one that took Odetta's legs.