She was born in Kampala, Uganda to Sarah Nabbutto and Prince Kizito Ssegamwenge. She went to Buganda Road Primary School, Kampala High School, and Nabingo College. She studied marketing at the Makerere University Business School Nakawa and Makerere University School of Music Dance and Drama.
Ndagire started The Next Ugandan Music, an American Idol-style show. Ndagire also started a new training workshop for filmmakers at her center, the Mariam Ndagire Film and Performing Arts Centre, which holds film workshops for filmmakers every year. Among the workshops' notable participants are Sarah Kisauzi Sentongo, an actress with Deception, the NTV Uganda TV series, and screenwriter Usama Mukwaya.