Shannon spent most of the war unsuccessfully patrolling the North Sea for German warships and commerce raiders.

The engines were designed to reach a total of 27,000 indicated horsepower (20,000 kW) and were intended to give a maximum speed of 23 knots (43 km/h; 26 mph).

Shannon proved to be the slowest ship in the class; during her sea trials on 3 December 1907 her engines reached 29,644 indicated horsepower (22,106 kW), but she only reached a speed of 22.592 knots (41.8 km/h; 26.0 mph) The ship carried a maximum of 2,060 long tons (2,090 t) of coal and an additional 750 long tons (760 t) of fuel oil that was sprayed on the coal to increase its burn rate.

[1] The ship's main armament consisted of four BL 9.2-inch Mark XI guns in two twin-gun turrets, one each fore and aft.

Her secondary armament of ten BL 7.5-inch Mark II guns were mounted amidships in single turrets.

[5] While fitting out in Portsmouth, Shannon was accidentally struck on 5 December 1907 by the battleship Prince George which had broken loose from her anchorage; both ships were only lightly damaged.

In January 1914, she relieved Indomitable as flagship of the 2nd Cruiser Squadron during exercises off the northwest coast of Spain.

[4] In October 1914, the ship was patrolling off the coast of Norway and almost intercepted the armed merchant cruiser SS Berlin on several occasions.