Raised in County Dublin, she became a member of the Roman Catholic religious congregation of the Missionary Sisters of the Holy Rosary (MSHR).
She joined the Holy Rosary Sisters in Killeshandra, County Cavan, in 1940, and after religious profession on 28 August 1942, she later qualified as a medical doctor at University College Dublin.
Mission hospitals and feeding centers were overwhelmed by sick and wounded civilians and soldiers, and she worked to save lives and console homeless orphans.
As she narrated to Liam Ryan of the Evening Herald, given the chance to leave at the outbreak of war, she and ten other Irish nuns of the Holy Rosary Order, decided to stay.
She inspired the Suburban Maternity Clinics project, following her invitation in 1982 to Donal Denham, Ireland's first diplomat to serve in Zambia, to visit the UTH, where he found "women giving birth out in the corridors on emergency trollies, some 25,000 of them in 1981".
Mona Tyndall was credited with the primary role in setting up this network of rural clinics with trained local personnel, which dispensed natural family planning methods, and eventually raised awareness about the dangers of HIV/AIDS.