After his brief stint as Mayor of Los Angeles, Stephens entered the realm of federal politics. In the 1910 elections, Stephens was elected as a Republican for the 7th congressional district to the U.S. House of Representatives. Due to redistricting, Stephens changed constituencies to the newly created 10th congressional district for the 1912 elections, which he also won. During this time period, Stephens increasingly identified himself as a member of the Progressive movement, becoming a member of the Progressive Party, led by former U.S. President Theodore Roosevelt and California Governor Hiram Johnson. Stephens was one of the 13 Progressives to be elected to the U.S. House of Representatives in the 1910s, four of which (including himself) came from California. He successfully defended his seat again in the 1914 elections, winning a consecutive third term to the House. Stephens would continue to identify himself as a member of the Progressive Party until the party's dissolution in 1916, when he rejoined the Republican Party.