Born Francis Donnelly on February 11, 1891 in Woodstock, Illinois, he was the son of John Donnelly and Julia Eddy. He attended the Passionist Preparatory Seminary for a while and entered the novitiate at Sacred Heart Retreat in Louisville, Kentucky. On August 1912 he was given the name Agatho. Novitiate completed, he professed his vows on September 15, 1913. On May 20, 1920 he was ordained to the priesthood.
He was assigned to St. Paul, Kansas. He had a brief term as Lector of English Literature and History at the Preparatory Seminary at Normandy, Missouri. He served several years as Director of Students at Cincinnati, Ohio. He also spent one year at the original Passionist foundation in Houston, Texas. For a good part of his ministry Father Donnelly operated out of St. Paul, Kansas where he did parochial ministry at St. Paul, Oswego, Chetopa, South Mound, and Erie, Kansas.
Beginning in 1947 he served as a retreat master at the Passionist retreat in Houston to religious sisters and clergy. He was a good community man. A man of substantial weight he was teased by his fellow Passionists but seemed to take it in good humor. However, fifteen years prior to his death he suffered from a nervous breakdown. He did recover.
He liked to smoke a pipe, fish, enjoyed literature and was a splendid writer. He often wrote for The Passionist. He always took time to explain Catholic teaching and possessed an excellent ability to recall names. Often times Father Donnelly assisted the brothers in the kitchen, especially after he had caught some fish!
On September 23, 1949, while substituting at St. Leo’s, Kansas, Father Donnelly finished his supper at the Dominican Sisters and died. He had been healthy and his sudden death was a shock. It appeared to be a heart attack. He was buried at St. Paul, Kansas.