William Dennison was born in Cincinnati in 1815. After graduating from Miami University he worked as a lawyer. Later he became a successful railroad promoter and bank president. A member of the Republican Party he was a strong opponent of slavery.

Elected governor of Ohio in 1859, Dennison was a advocate of firm action against the Confederacy after the capture of Fort Sumter. He was also an enthusiastic recruiter of soldiers for the Union Army. Although he shared the political views of Salmon Chase he supported the nomination of Abraham Lincoln in 1864.

In September, 1864, Dennison's loyalty was rewarded when he was appointed as Postmaster General. Dennison shared the views of the Radical Republicans and disillusioned with the increasingly conservative policies of President Andrew Johnson, he resigned from the Cabinet in 1866. William Dennison died in 1882.