Margaret had scarcely been delivered to the Canadian Customs department when, in August, 1914, she was taken up by the RCN for patrol work, chiefly in the St Lawrence River and Gulf of St. Lawrence. She was commissioned from February 3, 1915 to April 3, 1919, and soon afterward returned to the Dominion government. Sold to the Brazilian Navy about 1935 and renamed Rio Branco, she was discarded in 1958. (Source: The Ships of Canada's Naval Forces 1910-1981 by Ken Macpherson & John Burgess)
Following the end of the war, HMCS Margaret was returned to the Customs Preventive Service (CPS), and carried out her first patrols in the Gulf of St. Lawrence and on the East Coast in the spring of 1919. Converted from coal to oil burning in 1925, her armament was also reduced in 1927 to one six-pounder and seventeen rifles. During these years, much of her work consisted of customs patrols against rum running. In 1932, the CPS was absorbed by the Royal Canadian Mounted Police (RCMP), and the service's personnel and ships were transferred to the RCMP's Marine Section. Following the transfer of the CPS's responsibilities to the RCMP in 1932, the number of personnel was reduced and some former CPS vessels were discarded. Margaret was among the CPS vessels sold later that year as a result of these cutbacks, and was acquired by the Brazilian state of São Paulo, which named her Ruth, during the Constitutionalist Revolution of 1932. Confiscated by the Brazilian government, she was renamed Rio Branco and was converted to a hydrographic survey ship for the Brazilian Navy in 1934. After serving as a coastal escort vessel in World War II, the Rio Branco returned to hydrographic work and was discarded in 1957 or 1958.