He practiced law in Chicago, Illinois until 1917 and then moved to Keokuk, Lee County, Iowa in 1917.

From 1921 to 1927, Rankin served in the Iowa House of Representatives and was a Republican.

From 1925 to 1938, Rankin served as Iowa District Court judge for Lee County.

Rankin served as Iowa Attorney General from 1940 until his death in 1947.

Rankin died of lung cancer at a veterans hospital in Des Moines, Iowa.