Michael Cornelius O'Neill was born on February 15, 1898 at Kemptville, Ontario. After completing high school, he served in the Canadian forces in the First World War and was decorated with the Miltary Medal for bravery. After the war, he worked for three years as clerk for the Soldiers' Settlement Board. His earnings and solider's grants enabled him to attend St. Michael's College, Toronto.

After further studies at St. Augustine's Seminary, Michael O'Neill was ordained to the priesthood on December 21, 1927, and came out West.

He joined the staff at St. Joseph's Seminary, Edmonton, Alberta, a position he held until 1939. He then volunteered for overseas duty as military chaplain. He was demobilized in 1946, having attained the rank of colonel and chief Roman Catholic chaplain in the Canadian forces overseas.

Returning to Edmonton, he was appointed rector of St. Joseph's Cathedral until called to be the fourth Archbishop of Regina on December 4, 1947. Consecrated on April 14, 1948, he was installed in Regina on April 21.

Archbishop O'Neill had a long and fruitful ministry in the Archdiocese. The highlight of his term of office was his attendance at all the sessions of the Second Vatican Council (October 1962 to December 1965). Its decrees were well received, and vigorous renewal marked all areas of diocesan life.

In May 1973, Archbishop O'Neill celebrated 25 years of consecration in the episcopacy. He had attained his 75th birthday and received his sought-after retirement in September of that year.

He spent his next ten years in Regina, taking an active interest in the community. He died at age 85 on June 10, 1983.

The above portrait hangs in Holy Rosary Cathedral, Regina.