Assigned to the 2nd Division, Asiatic Fleet (formalized as the Yangtze Patrol in 1922), Monocacy sailed upriver from Shanghai on 29 June 1914 to cruise between Ichang and Chungking. For the next 15 years, but for annual visits to Shanghai for overhaul, the gunboat patrolled the upper Yangtze with Chungking her upriver base. Monocacy protected American interests in treaty ports down the entire length of the Yangtze, at times escorting vessels, evacuating American citizens during periods of disturbance, and, in general, assisting U.S. Consulates in various Chinese cities.

The annual springtime trip down the Yangtze for overhaul at Shanghai was timed to coincide with rising water through the Three Gorges. A sailor reaching the entertainments of Shanghai with six months' pay in his pockets from the time patrolling upriver claimed to use most of his money for liquor and women, and spend the rest foolishly.^[1]

On 16 January 1918, Chinese southern revolutionary troops attacked the little warship 50 miles (80 km) above Chenglin; during which Monocacy prevented the rebels from firing on a Japanese steamer standing down stream. One crewman was killed and two others were wounded in the short but fierce fight. In June 1920, she was reclassified as PG-20 and a month later she engaged in another battle. Three years later, in February and March 1923, the gunboat operated against bandits holding up American missionaries and firing upon U.S. flag vessels above Kiangnang, rescuing the Syracuse Medical Unit and a number of American families. Later that year, she protected U.S. river commerce from warlord armies and then in August, she established friendly relations with the Governor of Kiangsin Province.

Monocacy continued her patrol operations on the upper Yangtze, reclassified PR‑2 on 15 June 1928. She was placed in reserve 24 June 1929. Based at Shanghai, the gunboat cruised the lower river, making less frequent voyages to her original patrol area between Chungking and Ichang. She was placed in full commission once again 19 September 1931 to join other American naval vessels on the river in assisting the many Chinese forced from their homes by August floods which had inundated 34,000 square miles (88,000 km²) of land during the worst disaster in the river's history. In 1933, Monocacy began to serve as station ship in various treaty ports during the cruising season, with her crew serving as a landing force in case of trouble.

Monocacy was at Kiuklang protecting American neutrality during the Japanese invasion of China, when on 29 August 1938 several mines exploded within 80 yards (73 m) of the ship, showering the gunboat with fragments. She was then held at the port until the Japanese completed sweeping operations some days later. She was decommissioned at Shanghai on 31 January 1939. The veteran gunboat was towed to sea and sunk 10 February 1939 in deep water off the China coast.

• This article incorporates text from the public domain Dictionary of American Naval Fighting Ships. The entry can be found here.

• Photo gallery at navsource.org