Upon graduation, Kent returned to Chicago and entered the real estate and livestock businesses. He also became involved in politics, becoming a member of the city council and president of the Municipal Voter's League of Chicago.
In 1907, Kent returned to California and entered the national stage of politics by earning election as a Progressive Republican to the 62nd United States Congress. For the 63rd and 64th Congresses he was reelected as an Independent. In total, he served in Congress from March 4, 1911 to March 3, 1917.
Kent was also heavily involved in local politics; he was one of the major supporters of the creation of the Marin Municipal Water District in 1911, and also backed early efforts for a Golden Gate Bridge. While Kent supported conservation, he also actively worked to promote growth and development in Marin. His wealth as one of the major landowners in the county increased greatly as property values rose.
Following his career in Congress, Kent was appointed to the United States Tariff Commission (now known as the United States International Trade Commission). He served on the Commission from March 21, 1917 to March 31, 1920.
Kent was also a philanthropist. Together with his wife, Elizabeth Thacher Kent, he purchased 611 acres (2.5 km²) of one the last remaining stands of coast redwoods along Redwood Creek north of San Francisco Bay. To protect the redwood grove from development, he donated 295 acres (1.19 km²) to the Federal Government. President Theodore Roosevelt declared the area a national monument in 1908 and suggested naming the monument after Kent. Kent demurred and suggested the grove be named Muir Woods National Monument, after naturalist John Muir. Portraits of the Kent family by artist Herbert A. Collins hang there.