She was a nightclub lounge singer, but after she witnessed her mobster boyfriend commit murder she went into hiding at a run-down convent, passing off as one of the nuns. There she was given the name Sister Mary Clarence. She at first struggled with adapting to austere convent life. She often got herself into trouble with Reverend Mother, especially when she sneaked out of the convent to hang out at a bar. However, she befriends several of the nuns, including Sisters Mary Patrick, Mary Robert, and Mary Lazarus. She eventually grew accustom to convent life, and became the new choir director where she helped the nuns become better singers. Their improved performance, infused with gospel and rock and roll, helps improve the church's image to the public, bringing people in off the streets.
Eventually, Deloris is found and abducted by Vince's goons. Reverend Mother and the rest of the nuns set out to rescue her, even after learning the truth about her real identity. They find Deloris in Vince's casino, and together with the nuns thru attempt to confuse the mobsters while sneaking her out, but end up being trapped in the casino lounge. Not wishing to risk the lives of her friends, Deloris prepares to sacrifice herself. However, Vince and his men have difficulty bringing themselves to shoot Deloris while she is in a nun's habit. The delay is long enough for the police, led by Souther, to arrive and arrest Vince and his men. Despite being annoyed at the risk and other things Deloris did to the convent, the Reverend Mother thanks her for what she has done. Deloris is last seen orchestrating the choir in the refurbished Saint Katherine's, receiving a standing ovation from all, including the Pope.
Up to the sequel, Deloris becomes a famous performer in Las Vegas. During her latest performance, she is reunited with her nun friends, Sisters Mary Patrick, Mary Robert, and Mary Lazarus, who ask for her to reprise her persona as Sister Mary Clarence. St. Francis Academy (which Deloris attended as a child) is facing closure unless the school's reputation can be improved.
Deloris reluctantly agrees to help them. She returns as Sister Mary Clarence and becomes the new music teacher, where she turns a group of rowdy teenagers into a choir after they display spontaneous, synchronised singing. She even convinces ringleader, Rita Watson, to join the choir after she hears about her singing. Deloris and the students restore the school's old music room and start practicing, with Rita becoming the lead singer alongside Ahmal. Their performance receives a standing ovation by the school, and during the All-State Choir Championship they win first prize, saving St. Francis Academy from closure.
- Bette Midler was originally intended for the role of Deloris, but later turned down the role, fearing that her fans would not want to see her play a nun. Eventually, Whoopi Goldberg signed to play the lead.