During World War II he served in the United States Navy's amphibious forces.
He received his juris doctor from Northwestern University School of Law in 1946.
He was an active member of the Democratic Party in Stephenson County, Illinois.
[3] In 1978, Mahoney was elected a judge of the 15th Circuit Court, which included Jo Daviess, Carroll, Ogle, Lee, and Stephenson counties, a rare feat for a Democratic candidate.
[5] Eric S. DeMar of Galena, Illinois was appointed to the Mahoney vacancy.