Hagan was born in Nacogdoches, Texas, the son of Ike and Rebecca (Campbell) Parmley.

He was a farmer at the time he filled out his World War I draft card. He had married Tennie Martin (1889-1945) and their first child, Marie (1913-1967), had been born. Later they would add Hazel (1917-1951) and Samuel "Hagan" Jr. (1919-1966) to the family. The card goes on to characterize him as tall and slender, with gray eyes and brown hair.

In August 1942, he was elected sheriff of Nacogdoches County, ousting incumbent Clint Eddings by a 2,935-to-2,750 vote. He served from Jan. 1, 1943, until suffering a fatal heart attack Aug. 16, 1945. His son and namesake, a Marine sergeant stationed in the Panama Canal Zone was called home to fill the remainder of his father's term by the commissioners' court.