Educated at Marlborough College and Cambridge (Trinity College), Petherwick served with the Royal Devon Yeomanry at the outbreak of the First World War.  Invalided out in 1915, he served at the Foreign Office (1916 – 1917) but was sufficiently recovered to be recommissioned, except this time, into the Royal Scots Greys, in 1917 and he served in France until the War ended.

Petherwick unsuccessfully contested Penryn & Falmouth in 1929 but secured the seat in 1931, being re-elected in 1935. 

He wrote two novels, ‘Captain Culverin’ (1932) and ‘Victoire’ (1943) and served in the ‘Caretaker Government’ of 1945 as Financial Secretary to the War Office but lost the 1945 election to the Labour candidate, Evelyn King.

Post-War, he was a Director of the Prudential Assurance Co. Ltd. and  High Sheriff of Cornwall (1957) and became known for nurturing the gardens of his home, Porthpean House near St. Austell.