Born in St. Louis, Missouri, Cahill was in the United States Air Force from 1942 to 1946, and then received a Bachelor of Science degree from Saint Louis University in 1949.

He was a regional attorney for the United States Office of Economic Opportunity in Kansas City, Missouri from 1966 to 1968, and then the general manager of the Human Development Corporation in St. Louis from 1968 to 1972.

[2] On April 2, 1980, President Jimmy Carter nominated Cahill to a seat on the United States District Court for the Eastern District of Missouri vacated by Judge James Hargrove Meredith.

Cahill was confirmed by the United States Senate on May 21, 1980, and received his commission on May 23, 1980.

He assumed senior status on April 9, 1992, and served in that capacity until his death on August 18, 2004, in St.