He attended the University of Vermont and the Wharton School of the University of Pennsylvania,
McEwen graduated from Albany Law School in 1947, was admitted to the bar and practiced in Ogdensburg.
He was elected to the New York State Senate on January 5, 1954, to fill the vacancy caused by the appointment of Paul Doctorate. Graves to the New York Supreme Court. McEwen remained in the State Senate until 1964, sitting in the 169th, 170th, 171st, 172nd, 173rd and 174th New York State Legislatures. He was elected as a Republican to the 89th, 90th, 91st, 92nd, 93rd, 94th, 95th and 96th United States Congresses, holding office from January 3, 1965, to January 3, 1981.
He did not run for reelection in 1980.
In 1981 McEwen was appointed by Ronald Reagan to the International Joint Commission, the United States-Canada body that advises the governments of the respective countries on issues related to boundary waters, and he served until 1989. In 1982, the Custom House at Ogdensburg was named in his honor.
McEwen died in Ogdensburg on June 15, 1997. He was buried in Ogdensburg Cemetery.