Kenneth Francis Thesing was born on May 9, 1942, in Lewiston, Minnesota, son of Theodore John and Evelyn Regina Thesing. He has nine brothers, one of whom is Father Gilbert Thesing, O.P. working since 1968 in Nigeria, West Africa. The others are married with their own families. Ken attended the public schools in Lewiston and graduated from high school in June 1960. He completed one year of studies at St. Mary's College in Winona, Minnesota. On September 6, 1961 Ken entered Maryknoll College, Glen Ellyn, Illinois. He became the Editor of the College publication called "DuPage", and received his Bachelor of Arts degree in Philosophy on June 7, 1964. At Maryknoll Seminary in Ossining, New York, he received his Bachelor of Divinity degree in June 1968, and his Master of Arts degree in Theology in May 1969. He was ordained a priest at Maryknoll Seminary on May 24, 1969.
Following ordination, Father Thesing was assigned to the Development Department and served for three years in the Minneapolis Development House. He was then assigned to the Maryknoll Mission in Tanzania, East Africa. After language and cultural studies Father Thesing was placed in the rural parish of Ndoleleji. His work entailed basic parochial ministry, the training of lay leaders and the promotion of personal and civic growth among the people through various projects in agriculture, health and education. In 1974 he was appointed Coordinator of the Agricultural Program in the Shinyanga Diocese.
Through much of his ministry during the passing years, Father Thesing was also closely associated with matters of administration. On August 6, 1975 he was appointed First Assistant to the Regional Superior for a term of three years. In February 1978 he was elected First Delegate for the Africa Region to the Seventh General Chapter at Maryknoll, New York, in 1978. In May 1978 he was appointed Second Assistant to the Regional Superior.
Upon his return to Shinyanga he was appointed First Assistant to the Tanzania Regional Superior in February 1979 for three years. When the Africa Area was formed after the 1978 General Chapter, Father Thesing served as the first Africa Area Coordinator. In 1980 Father Thesing was assigned to the Gula Parish and part of his work entailed developing the new quasi-parish of Mwabusalu. On April 1, 1981, Father Thesing was appointed Assistant Regional Superior of the Tanzania Mission Region. In 1982 he was transferred as Pastor to the Queen of Apostles Parish in Ndoleleji. During home furlough in 1982 he attended the Spiritual Renewal Program.
Father Thesing was elected Second Delegate for the Tanzania Region for the Eighth General Chapter in October-November of 1984. During the Chapter he was elected Second Assistant General and served for six years on the General Council. At the Ninth General Chapter in October-November 1990, Father General was elected Superior General of the Maryknoll Society.