Promoted to lieutenant, after first classes in every subject and maximum seniority, on 14 February 1889,[8] Brock joined the battleship HMS Trafalgar, flagship of the Second-in-Command of the Mediterranean Fleet, in April 1890.[7] After attending the gunnery school HMS Excellent, he became gunnery officer in the turret shipHMS Devastation at Devonport in August 1894.[7] He went on to be gunnery officer in the cruiserHMS Cambrian in the Mediterranean Fleet in October 1894 and gunnery officer in the battleship HMS Ramillies, flagship of the Commander-in-Chief of the Mediterranean Fleet, in November 1895.[7] Promoted to commander on 1 January 1900,[9] he became executive officer in the battleship HMS Repulse in the Channel Squadron in January 1901 and executive officer in the battleship HMS Renown, flagship of the Commander-in-Chief of the Mediterranean Fleet, in August 1901.[7] In July 1902 it was announced that he was appointed to HMS Albion, second flagship of the China Station,[10] but the appointment was cancelled the following week.[11] He was briefly posted to HMS Empress of India, serving in the Home Fleet, in early November 1902,[12] but in January 1903 he became commanding officer of the despatch vessel HMS Alacrity, serving on the China station.[13]
Promoted to captain on 1 January 1904,[14] Brock left the Alacrity after a year in January 1904, and became commanding officer of the newly commissioned Admiralty yachtHMS Enchantress in May 1904. He subsequently became Flag Captain to the Commander-in-Chief of the Mediterranean Fleet in the battleship HMS Bulwark in May 1905.[7] He went on to be assistant director of Naval Intelligence at the Admiralty in 1906 and then became Flag Captain to the Vice-Admiral commanding the Second Division of the Home Fleet in the battleship HMS King Edward VII in March 1909, before returning to the Admiralty as assistant director of Naval Mobilisation in August 1910.[7] After that he became commanding officer of the battlecruiserHMS Princess Royal in August 1912.[7] He was appointed an aide-de-camp to the King on 24 October 1913.[15]
Brock became Deputy Chief of the Naval Staff and a Lord Commissioner of the Admiralty in July 1919 with promotion to vice admiral on 3 October.[23] He went on to be Commander-in-Chief of the Mediterranean Fleet with his flag in the battleship HMS Iron Duke in April 1922.[18] Following the Turkish victory in Anatolia at the end of the Greco-Turkish War in August 1922, Brock organised the rescue of the fleeing Greek army and, by skilful deployment of his ships, he dissuaded the advancing Turks, led by Mustafa Kemal Atatürk, from attacking the British garrison at Chanak in the Dardanelles neutral zone in September.[18] By 12 September 1922, during the sack of Smyrna by the Turkish army, there were more than 150,000 refugees gathering on the quayside of Smyrna. While anchored in the Bay of Smyrna and appalled by the scenes of the refugees being attacked by Turkish soldiers, Brock ordered the ship's band to play music in order to drown out the sound of the screaming people. Later, Brock took dinner with his senior officers in the captain's quarters.[24] For his diplomatic handling of the Chanak Crisis, Brock was commended by Leo Amery, the First Lord of the Admiralty, in the House of Commons in 1923.[18] Promoted to full admiral on 31 July 1924,[25] he moved his flag to the battleship HMS Queen Elizabeth later that year.[18]