Mar Theodosius studied at Baselious College, Kottayam and Mar Thoma College, Thiruvalla. After taking a degree in Science he joined Leonard Theological College and took BD Degree in 1972. His ordination as ‘Kasseesa’ was in the year 1973. In 1980, he took a Master’s Degree in Comparative Religions from Visva Bharathi University and Doctorate from Mc Master University, Hamilton, Canada in 1986. He was consecrated as Episcopa on December 9, 1989 along with Mar Athanasius and Mar Coorilos.

From 1973 onwards, he was in charge of several parishes including Mumbai, Calcutta, Toronto and Nanthancode and he served as Director of Thomas Mar Athanasius Orientation Centre, Manganam. On consecration as Episcopa he was given charge of Kunnamkulam – Madras Diocese. He was the Episcopa of the Chennai – Bangalore and Malaysia – Singapore – Australia Dioceses. He is presently the Diocesan Bishop of the Mumbai and Ranni-Nilackel Diocese.

On July 12th 2020, he was given as the title, The Suffragan Metropolitan, during the service at Poolatheen Chapel officiated by the Metropolitan The Most Rev. Dr. Joseph Mar Thoma in the presence of the Bishops, Clergy and laity of the Church.

On November 14, 2020, Tirumeni was installed as the 22nd Mar Thoma Metropolitan with the title Theodosius Mar Thoma Metropolitan. The Installation Service was held at Dr. Alexander Mar Thoma Valiya Metropolitan Smaraka Auditorium, Tiruvalla.