Hjalmarson was the son of Major General Harald Hjalmarson, who helped to build up the Persian Government Gendarmerie and served in the Finnish Civil War. The elder Hjalmarson committed suicide after his return from Finland in 1918. Jarl Hjalmarson's mother was Blenda Hjalmarson, born Lindeborg. In 1929 Hjalmarson received a bachelor's degree in law. He married Eywor Dahlén in 1933 and had three sons, Torgils (born in 1936), Staffan (born in 1939) and Bo (born in 1943).
