The remaining four guns amidships were raised to the extended forecastle deck, which meant that they could be worked in all weathers.

[6] After the two German ships avoided the British forces and reached Turkey, Chatham was detached for operations in the Red Sea on 13 August 1914.

Chatham arrived at Zanzibar on 28 September, but her participation in the search was delayed when, during the night of 1 October, cruising off Mombasa, she ran aground on the Leven Reef, just to the northward of the entrance to Kilindini Harbour.

[12] On 7 November Chatham hit Somali with a shell, causing a fire that destroyed the supply ship, while on 10 November the British scuttled the collier Newbridge in the main channel of the Delta, blocking Königsberg from escaping to sea.

[5] On 12–13 July 1915 she provided gunfire support to an attack along the Achi Baba Nullah dry water course on Cape Helles,[15] and on 6–7 August took part in the Landing at Suvla Bay, acting as the flagship of Rear-Admiral John de Robeck, in command of Naval Forces during the operation.

[16] On 20 December Chatham acted as the flagship for Admiral Weymss during the evacuation from Suvla Bay and Anzac Cove.

[17] In 1916 she returned to home waters and joined the 3rd Light Cruiser Squadron of the Grand Fleet.

After the war, Chatham was lent to the New Zealand Division of the Royal Navy from 1920 to 1924,[5][18] She proceeded via the Royal Naval Dockyard in the Imperial fortress colony of Bermuda (home base of the 8th Cruiser Squadron on the North America and West Indies Station), before cruising to the West Indies and becoming the first Royal Naval vessel from Bermuda to pass through the Panama Canal in December, 1920 (the geographic limits of the station controlled from Bermuda had grown over the preceding century from the western North Atlantic to absorb the area of the Jamaica Station, and following the first World War would absorb the former areas of the South East Coast of America Station and, utilising the canal, the Pacific Station, demonstrating the amity and the convergence of national interests between the United Kingdom and the United States).