When World War I began in August 1914, she was assigned to the 9th Cruiser Squadron in the Central Atlantic to intercept German commerce raiders and protect Allied shipping.

She then sank the German armed merchant cruiser SMS Kaiser Wilhelm der Grosse off the coast of Spanish Sahara.

She was powered by two 4-cylinder triple-expansion steam engines, each driving one shaft, which produced a total of 10,000 indicated horsepower (7,500 kW) designed to give a maximum speed of 20 knots (37 km/h; 23 mph).

[6] She was completed on 7 December 1899[1] and commissioned by Captain Frederic Brock temporarily for the Training squadron,[7] which took her to Gibraltar in March 1900.

[8] In February 1900 she received her first commissioned, to serve in the Indian Ocean as the flagship of Rear-Admiral Day Bosanquet, the Commander-in-Chief, East Indies,[9] based at Trincomalee.

She was at Mauritius in August 1902 where she took part in local celebrations for the Coronation of King Edward VII and Queen Alexandra,[12] and in November that year visited Colombo.

Fighting broke out at 15:10, and lasted until 16:45, when the crew of Kaiser Wilhelm der Grosse abandoned ship and escaped to the shore.

The German ship was sunk, with the British losing one man killed (Richard James Lobb) and five injured in the engagement.

[19] On 15 October Highflyer briefly became the flagship of the Cape Verde station, when Stoddard was ordered to Pernambuco, Brazil.

Later in the same month she was ordered to accompany the transport ships carrying the Cape garrison back to Britain and then searched the Atlantic coast of North Africa for the German light cruiser SMS Karlsruhe.

[20] Highflyer then took part in the search for the commerce raider Kronprinz Wilhelm, coming close to catching her in January 1915.

[22] This was the period of unrestricted submarine warfare, and the Admiralty eventually decided to operate a convoy system in the North Atlantic.

Highflyer launched a whaleboat before the explosion to investigate the fire aboard Mont-Blanc; the ship exploded before they reached her, killing nine of ten men in the boat.