"Lark", like the rest of the "Laforey" class, joined the Harwich Force at the outbreak of the First World War. On 5 August 1914, the Third Destroyer Flotilla, led by the light cruiser and including "Lark", carried out a sweep to prevent German minelayers or torpedo craft entering the English Channel. Later that morning, the flotilla leader and sister ship were ordered ahead of the flotilla to investigate, and came across the German minelayer , laying mines off Southwold on the Suffolk coast. The German ship attempted to escape to neutral waters, but was engaged and sunk by "Lance", "Landrail" and "Amphion". The flotilla was returning from the sweep in 6 August when it ran into the minefield laid by "Königin Luise". "Amphion" struck a mine and began to sink, her crew abandoning ship, but drifted onto another mine, with debris from the resulting explosion hitting "Lark" killing two of "Amphion"s crew that had just been rescued as well as a prisoner from "Königin Luise". In total, 151 of "Amphion"s crew were killed. On 28 August 1914, the Harwich Force, supported by light cruisers and battlecruisers of the Grand Fleet, carried out a raid towards Heligoland with the intention of destroying patrolling German torpedo boats. "Lark" formed part of the 2nd Division of the Third Flotilla during this operation. "Lark" took part in torpedo attacks against the German light cruisers , and , expending all her torpedoes and 4-inch ammunition, requiring her to borrow 100 rounds of 4-inch ammunition from .