He served in the Georgia House of Representatives from 1939 to 1944, resigned from office to join the United States Army as a member of the Signal Corps for two years.

Between 1953 and 1960, Hagan served in numerous other state government positions and also worked in the insurance and financial planning industry and several agricultural pursuits.

In 1960, Hagan challenged incumbent Congressman Prince H. Preston, Jr., a noted segregationist in the Democratic primary for Georgia's 1st congressional district.

Preston narrowly prevailed in the actual popular vote (34,318 to 33,629), but under Georgia's election laws, Hagan defeated Preston on "county unit" votes by a margin of 26 to 18 and went on to win unopposed in the general election (the Georgia Republican Party being very weak at the time and fielded very few candidates, having not won a race for any congressional race since 1872).

In 1972, former Hagan staffer and assistant Ronald 'Bo' Ginn would aggressively challenge his former boss in a spirited three-man primary.