Augustus Agar was born in Kandy, Ceylon, on 4 January 1890. He was the thirteenth child of John Shelton Agar/Eagar, an Irishman from Milltown, County Kerry, who had left his native land in 1860 to become a successful tea planter in Ceylon. John's cousin Honora Eagar was the first wife of Jeremiah O'Donovan Rossa himself an Irish Fenian leader and member of the Irish Republican Brotherhood. Agar was brought up in comfortable circumstances in a fine house with servants. Agar's mother, who was Austrian, died shortly after his birth, and at the age of eight he was sent with one of his brothers to school in England. All his brothers were educated in English public schools, and all his sisters were educated in Austrian or German schools. His father died in 1902 of cholera which he had caught during a visit to China.