Dumas was born in Mount Barker, South Australia, the fourth child and youngest of three sons of Amelia Dumas, née Paltridge, (died 1 November 1938) and Charles M. R. Dumas (1851-1935), who founded the Mount Barker Courier.
He was educated at Mount Barker and Victor Harbor and in 1904 won a scholarship to the Teachers' College [REF].