He was a member of the faculty of Northwestern University Pritzker School of Law from 1939 to 1942.

He was in the United States Naval Reserve during World War II, from 1942 to 1945, returning to private practice in Washington, D.C. from 1946 to 1948, and to the Northwestern University Pritzker School of Law faculty from 1948 to 1949.

He was a counsel to Illinois Governor Adlai E. Stevenson from 1949 to 1953, taking up private practice in Chicago, Illinois from 1953 to 1963, including service as general counsel to the Chicago and North Western Railway from 1957 to 1963.

[2] On January 15, 1963, McGowan was nominated by President John F. Kennedy to a seat on the United States Court of Appeals for the District of Columbia Circuit vacated by Judge Henry White Edgerton.

[3][4] McGowan assumed senior status on August 31, 1981, and served in that capacity until his death on December 21, 1987, in Washington, D.C.[2]