Smyllie was educated at Sligo Grammar School and entered Trinity College, Dublin in 1912.
Working as a vacation tutor to an American boy in Germany at the start of World War I, he was detained in Ruhleben internment camp, near Berlin, during the war.
On returning to Ireland, he reported on the Versailles Treaty for The Irish Times, then edited by John E. Healy.
He established a non-partisan profile and a modern Irish character for the erstwhile ascendancy paper; for example, he dropped "Kingstown Harbour" for "Dún Laoghaire".
He was assisted by Alec Newman and Lionel Fleming, recruited Patrick Campbell and enlisted Flann O'Brien to write his thrice-weekly column "Cruiskeen Lawn" as Myles na gCopaleen.