Frothingham was elected a member of the Massachusetts House of Representatives, and served as Speaker from 1904 to 1905. He was the Republican nominee in the 1905 Boston mayoral election after narrowly defeating former Judge Henry S. Dewey. He lost the general election to Democrat John F. Fitzgerald 48% to 39%. He served as the 41st Lieutenant Governor 1909–1911, but was an unsuccessful candidate for Governor in 1911. He was lecturer at Harvard. He then moved to North Easton and continued the practice of law. He was a delegate to the Republican National Convention in 1916.