He was born in New South Wales, Australia. He started working at an early age as gold miner in Australia. In 1903 he was involved in a miner’s strike in Victoria Australia. The strike was defeated and Semple ended up being blacklisted.
To avoid the black list Semple moved to the West Coast of the South Island of New Zealand. By 1907 he was president of the Runanga Miner’s Union and earned himself nick name 'Fighting Bob Semple'.
He was jailed in 1913 for supporting the general strike and again in 1916 after fighting conscription for overseas service during World War I.
Semple was elected to Parliament for the Labour Party in 1918. He won the Wellington East seat in 1928 and held it until he retired in 1954 (although it was renamed to Miramar while he held it).
He did not seek re-election in the 1954 election and died in early 1955.