[6] After successfully undergoing trials[7] where she reached a speed of 34.619 knots (64.114 km/h; 39.839 mph),[2] Cossack was commissioned in April 1908.

[6] Shortly after Cossack entered service, it was decided to strengthen the armament of the first batch of Tribals by adding another two 12 pounder guns, this being done in 1909.

[4][8] Cossack was considered by her Captain to be a poorer sea boat than River-class destroyers,[b] but a better gun platform.

[15] In October 1914, Cossack was one of a number of warships of the Dover Patrol that were deployed to help support Belgian ground forces during the Battle of the Yser, with all available ships being used to carry out shore bombardment operations.

At one stage, on 20 October 1914, after the destroyer Amazon was damaged by German shellfire, Rear Admiral Horace Hood transferred his flag to Cossack.

[16][17] On the night of 26/27 October 1916, German torpedo boats of their Flanders Flotilla carried out a large scale raid into the English Channel, hoping to attack the drifters watching the anti-submarine nets of the Dover Barrage, and to sink Allied shipping in the Channel.