Soon after his departure from Congress, U.S. Representative James Brooks died, leaving a vacant seat along the East River, stretching from the Lower East Side to Murray Hill. Cox consented to be nominated in the special election, and won in a landslide. Ultimately, Cox did not miss a single day of the Congressional session. Along with Pennsylvania Representative Samuel J. Randall, he led the opposition to the civil rights for freedmen. He was returned to the House from a slightly redrawn Lower East Side district in a landslide in 1874.