During the early stages of World War I, Sydney was involved in supporting the Australian Naval and Military Expeditionary Force, and escorting the first ANZAC convoy.

During 1915 and 1916, Sydney operated on the North America and West Indies Station, before joining the 2nd Light Cruiser Squadron at Greenock, Scotland in November 1916.

After the war's end, Sydney spent a year in reserve before being reactivated to serve as Flagship of the RAN.

[2] When World War I started, Sydney was north-bound to join Admiral George Patey and the battlecruiser HMAS Australia.

[2] The ships were quickly assigned to protect the Australian Naval and Military Expeditionary Force, which was used to capture German colonial assets in the region; Sydney participated in operations against Rabaul and Anguar Island in September.

Before capture, the station was able to transmit an SOS, which was received by the troop convoy, and Sydney was ordered to investigate.

Emden's wireless operators had overheard the distress call and the orders to Sydney, and prepared to meet the Australian warship.

[2] Sydney's first indication of Emden's location was when the German ship began to fire at a range of 6 nautical miles (11 km; 6.9 mi).

The Germans were still flying their war ensign, but pulled it down after Sydney transmitted an instruction to surrender, then fired two salvos when no response was forthcoming.

[7] 134 German personnel were killed, with the rest of the ship's company were captured by Sydney (apart from a shore party, which commandeered the schooner Ayesha and escaped) and were delivered to British forces at Valletta, Malta.

After leaving Malta, the Australian cruiser proceeded to join the North America and West Indies Station, arriving in Bermuda on 6 January 1915.

[5] On 9 September 1916, the cruiser sailed for the United Kingdom, and after a brief refit in Greenock, joined the 5th Battle Squadron at Scapa Flow.

[5] On 4 May 1917, while patrolling in the North Sea with the cruiser Dublin and the destroyers Nepean, Obdurate, Pelican and Pylades, Sydney took part in a battle with Zeppelin L43.

[9] The Camel chased the two aircraft for 60 miles (97 km) and shot down one, but was then forced to break off when one gun jammed and the other ran out of ammunition.

[19] Part of the bow, including the stem head, jackstaff, and fairleads, was set into the seawall at Milsons Point, under the Sydney Harbour Bridge.

[22] In 2014 a shield removed from HMAS Adelaide during a refit in 1943 and dumped on a tip on the Mornington Peninsula, Victoria, was transported to Perth for refurbishment.