Beck combined his legal career with a career in public service. He served as Assistant United States Attorney for the Eastern District of Pennsylvania 1888-1892, and as United States Attorney 1896-1900. Although he started out as a "Cleveland Democrat," he joined the Republican Party in 1900 and was subsequently appointed Assistant Attorney General by President William McKinley. In this capacity, he became involved with litigation concerning the government's regulatory powers, which reflected the wish of the administration to assist the American business community. Beck resigned in 1903, when he joined the New York law firm of Shearman and Sterling. He continued his law practices in New York, Philadelphia and Washington until 1921. In that year President Warren G. Harding appointed him Solicitor General of the United States. He resigned in 1925, briefly returned to his law practice and then was elected as a Republican to Congress in 1927, to fill the vacancy of James M. Hazlett. As a Congressman he was the leading spokesman in the campaign against Prohibition and he tried to fight the principles and legislation of the New Deal. Reelected three consecutive times, he resigned in 1934.

Having many personal contacts in England, Beck felt very strongly about the Allied cause and was one of the first Americans to make a case for the Entente, the alliance between Great Britain, France, and Russia prior to World War I. His most famous book, The Constitution of the United States (1924), sold over fifty thousand copies, including translations in German and French.

Beck was a devoted member, and later President, of the Philadelphia Shakespeare Society from 1913 until his death in April 1936. He married Lilla Lawrence Mitchell in 1890, and had a son and a daughter, James Montgomery Beck, Jr. and Beatrice.