Sister Mary Robert always wanted to be a nun, as she always wanted to lead a life of service. However, she has had this ambition to do more, which is something from herself. Because of being insecure, she always appears to be second place behind everyone else. She was also insecure about singing in the convent choir, and would lip sing at the back of the group. But with some help from Sister Mary Clarence who took over as the choir director, she was able to sing at the top of her voice.
Deloris Van Cartier's true identity was kept a secret from the other nuns, including Sister Mary Robert. Only Reverend Mother knew who she really was while everyone else referred her as Sister Mary Clarence. Mary Robert befriended Mary Clarence, as did Sisters Mary Patrick and Mary Lazarus. One night, she came into Mary Clarence's room, to giver her, her daisy alarm clock to help her get up on time. She also followed Mary Clarence when she sneaked out to hang out at a bar, tailed by Sister Mary Patrick. She assumed that she was ministering to the winos, which she stated earlier, and thought she could help. Mary Clarence tries to get them to leave, but Sister Mary Patrick wanted to party for a bit. By the time they got back to the convent, they were caught by Reverend Mother, who has Mary Clarence join the choir as punishment. During the day, Mary Robert and the other nuns volunteered Mary Clarence to take over as choir director, after learning of her music background. She helped rearrange the choir and train the nuns to become better singers. She even helps Mary Robert sing at the top of her voice.
During the Sunday Morning, Mary Robert sang "Hail Holy Queen" as the leading choir alongside Sister Mary Patrick. Their new improved performance started to amaze parishioners and visitors with their music and soon helped transform the neighborhood. However, Vince finds out where Deloris is and has his goons abduct her, along with Mary Robert. However, Deloris helps her escape and warns the other nuns. Reverend Mother reveals the truth of Mary Clarence's real identity to her and the other nuns. Despite knowing the truth, Mary Robert and the nuns do not care less about who she really is, as she has helped them so much and would be lost without her. So they journey to Vince's casino and rescue Deloris themselves.
Afterwards, Mary Robert, along with the choir, sings "I Will Follow Him" to a packed audience in the refurbished Saint Katherine's, with Deloris leading. Their performance receives a standing ovation from all, even from a special guest, the Pope.
In the sequel, Sister Mary Robert, along with the other nuns, now work as teachers at the St. Francis Academy in San Francisco, which is on the verge of being closed down unless the school's reputation can be improved. On Reverend Mother's request Mary Robert, along with Sisters Mary Patrick, and Mary Lazarus journey to Las Vagas, to find Deloris and convince her to help them save the school. Deloris reluctantly agrees to help and reprises her persona as Sister Mary Clarence.
When Mary Clarence starts off a school choir with a group of rowdy teenagers, Mary Robert overhears a teenage girl named Rita Watson singing. She gets Mary Clarence to convince her to join the choir and she does, becoming the lead choir along with Ahmal. During the All-State Choir Championship, Mary Robert is backstage with Sister Mary Patrick and Mary Lazarus, watching the school choir perform and win, saving the school from closure.
- Sister Mary Robert's singing voice was performed by Andrea Robinson, who would later on provide the singing voice of Queen Athena in The Little Mermaid: Ariel's Beginning.