The ship was placed in reserve in March 1906 and recommissioned in 1909 for service with the 4th Cruiser Squadron on the North America and West Indies Station.

Essex spent most of the first half of the war in the Atlantic Ocean, escorting convoys and searching for German commerce raiders.

The Monmouths were intended to protect British merchant shipping from fast cruisers like the French Guichen, Châteaurenault or the Dupleix class.

The engines produced a total of 22,000 indicated horsepower (16,000 kW) which was designed to give the ships a maximum speed of 23 knots (43 km/h; 26 mph).

[2] The Monmouth-class ships' main armament consisted of fourteen breech-loading (BL) 6-inch (152 mm) Mk VII guns.

[13] They were not satisfied by the Mexican apologies and U.S. President Woodrow Wilson ordered the city occupied in retribution and to forestall a major arms delivery to Victoriano Huerta's forces.

[14] The Mexicans resisted and stray bullets hit Essex the next day, wounding one man who was shot in both feet.

The following month, Essex ferried the Governor General of Canada, Prince Arthur, Duke of Connaught and Strathearn, to Newfoundland and Labrador before returning to Quebec City on 20 July.

[13] When Craddock received the preliminary war warning on 27 July, he ordered Essex to join her sister Lancaster in Bermuda, which she reached three days later.

As the Germans appeared to be concentrating their efforts in the Caribbean, Craddock ordered Essex south to reinforce his forces there in early September.

[16] The ship continued to patrol the sealanes from the Caribbean Sea to Canadian waters until the end of February 1915,[13] when she escorted a troop convoy from Halifax to Queenstown, Ireland.

Rear-Admiral Archibald Moore, commander of the 9th Cruiser Squadron, hoisted his flag aboard the ship on 4 September and pulled it down on the 29th.