William J. Cornwell (August 24, 1809 in Stanford, Dutchess County, New York – 1896) was an American lawyer and politician from New York.
He was the son of Eden Burroughs Cornwall (1766–1833) and Thankful (Harrison) Cornwall (c. 1765–1847). He married Marietta Sheldon, and they had three children.
He practiced law in Weedsport, New York.
He was a member of the New York State Assembly (Cayuga Co.) in 1846 and 1847.
He was a member of the New York State Senate (24th D.) in 1848 and 1849.
He was a Canal Appraiser from 1850 to 1855, and Auditor of the Canal Department from 1855 to 1856.