Hartshorn was President of the South Wales Miners' Federation, and a member of the National Executive of the Miners' Federation of Great Britain.

He was elected as Member of Parliament for Ogmore at the 1918 general election. He served as Postmaster-General in 1924, made a Privy Counsellor the same year, and became a member of the Simon Commission. In 1930, he was appointed Lord Privy Seal, serving until his death on 13 March 1931.