McKinley won election to the U.S. House of Representatives in 1876, defeating Democrat Leslie L. Sanborn. He won reelection twice more before losing a razor-thin race to Democrat Jonathan H. Wallace, who successfully challenged an original eight vote lead for McKinley.

He returned to the House after defeating D. R. Paige in an 1884 election. Once again, he earned reelection twice before losing to John G. Warwick in an 1890 election.