Born on the Isle of Wight to Irish parents, Clarke’s father was a sergeant in the British army. As a young man, he joined Clan na Gael and served 15 years in British jails for his role in a bombing campaign in London. He was released in 1898, and spent nine years in America. He returned to Dublin in 1907 setting up a tobacconist’s shop on Great Britain Street (now Parnell Square), before being co-opted onto the IRB Military Council which was responsible for planning the Rising.
Clarke was also Chairman and a Trustee of the Wolfe Tone Memorial Committee, which organised the first pilgrimage to his grave at Bodenstown, Co Kildare in 1911.
The first signatory of the Proclamation of Independence because of his seniority and life-long devotion to the cause of Irish independence, Clarke was with the group that occupied the GPO. He married Kathleen Daly, niece of the veteran Fenian John Daly, and had three children.
Clarke faced the firing squad at Kilmainham on May 3, 1916, age 59.