He was made the country's Attorney General in 1905, being appointed an Irish Privy Counsellor, and in 1916 became Lord Chief Justice of Ireland.

Mark Sturgis, the Dublin Castle official whose diaries give a vivid picture of these particular years of British rule, condemned Campbell bitterly as a coward who "does nothing and apparently thinks of nothing but the best way to show Sinn Féin that he is neutral and passive".

Irish historian R. B. McDowell comments that neither Sturgis nor Ross intended to stay in Southern Ireland when their Castle appointments ended, respectively returning to England and relocating to County Tyrone via London.

[6] In 1922 he was nominated to the new Free State Seanad by W. T. Cosgrave, and was elected by almost all of his fellow senators as its first Cathaoirleach (chairperson) on 12 December 1922.

[7] This was in the midst of the Irish Civil War and shortly after his appointment his family home in Kimmage, Dublin was burnt by the anti-Treaty IRA, as part of their campaign against the representatives of the new state.

The Dáil Courts were declared to have been illegal, but their outstanding 'judgements' were conferred with legal standing by a separate Act of the Oireachtas.

He was a longtime captain of one of the panels in the BBC gameshow Call My Bluff against British comedy writer Frank Muir.