Born in New Castle, Delaware in 1817, he attended the University of Delaware as a student of civil engineering. In addition to his engineering work, he studied law and was admitted to the Delaware Bar in 1848, beginning his practice in Wilmington. He was named as a commissioner to retrace the Mason-Dixon Line in 1849.

From 1849 to 1850, Riddle served as a Deputy Attorney General of the United States. He was elected as a Democrat to the Thirty-Second and Thirty-Third Congresses and served as the Representative from Delaware from March 4, 1851 to March 3, 1855. During the Thirty-Third Congress, Riddle was the chairman of the Committee on Engraving. He was defeated in a re-election bid in 1855 by Elisha Cullen.

During the Civil War, Riddle served with the Home Guard of Wilmington, Delaware. In 1864, he was elected to the United States Senate to fill the vacancy caused by the resignation of James A. Bayard and served from February 2, 1864, until his death in Washington, D.C., on March 29, 1867. He is interred in the Wilmington and Brandywine Cemetery, Wilmington, Del.

Political Offices Preceded by: John W. Houston U.S. Representative from Delaware (at-large) March 4, 1851 - March 3, 1855 Succeeded by: Elisha D. Cullen Preceded by: James A. Bayard, Jr. U.S. Senator from Delaware (class 1) February 2, 1864 - March 29, 1867 Succeeded by: James A. Bayard, Jr.