Molteno was born in Cape Town, then part of the Cape Colony; his father was John Charles Molteno who at the time was Prime Minister of the Cape. His father died when he was 14 years old, and he left the colony to enter the Royal Navy as a cadet. Quickly distinguished as a gun specialist, in 1893 he took part in the Vitu Expedition to Zanzibar as a lieutenant to crush the slave trade and was awarded the General African Medal. His effectiveness and ability on this mission were mentioned in despatches, and led to his being promoted to Commander. Molteno was later promoted to flag captain in 1913 and vice-admiral in 1926.World War 1:
He commanded several warships in World War 1, including HMS Revenge, Antrim, King George V, Shannon, Minotaur, Bellerophon and HMS Redoubtable, leading the bombardment of German bases along the Flanders coast. He also served as Flag-Captain of the 2nd Battle Squadron and the 3rd Cruiser Squadron.Molteno fought at the Battle of Jutland, the largest naval engagement of the First World War, commanding from the Armoured cruiser HMS Warrior.