Earlier in his career he served as United States Attorney for the Western District of Wisconsin during the presidency of John F. Kennedy.

Heffernan served in the Navy during World War II, and attended the Harvard Graduate School of Business.

At that time, Heffernan resigned his duties from his law firm and became Deputy Attorney General of Wisconsin, and remained in that position until 1962 when he was appointed as a United States Attorney for the Western District of Wisconsin by President John F.

The 31 years that Heffernan spent on the Supreme Court make him the third longest-serving judge in Wisconsin history.

[citation needed] Heffernan taught summer courses on appellate administration and opinion writing at New York University School of Law and was an adjunct professor of appellate practice and procedure at the University of Wisconsin Law School for fifteen years.