Riordan was elected as a Democrat to the 56th United States Congress, holding office from March 4, 1899, to March 3, 1901. He was a member of the New York State Senate (10th D.) from 1903 to 1906, sitting in the 126th, 127th, 128th and 129th New York State Legislatures.
Riordan was elected to the 59th United States Congress to fill the vacancy caused by the resignation of Timothy D. Sullivan and on the same day was elected to the 60th United States Congress. He was re-elected to the 61st and to the seven succeeding Congresses, holding office from November 6, 1906, until his death in Washington, D.C. on April 28, 1923. He was buried at the Calvary Cemetery in Queens.
- United States Congress. "Daniel J. Riordan (id: R000264)". Biographical Directory of the United States Congress.