William Dennison Jr. was born in 1815 in Cincinnati, Ohio. He graduated from Miami University in 1835, studied law, was admitted to the state bar in 1840, and then established a law practice in Columbus, Ohio.

From 1848 to 1850, Dennison, a Whig, served in the Ohio state senate, where he worked with Democrats to repeal the “Black Codes” that denied African-Americans their civil rights. Dennison resumed his law practice in 1850 but rejoined politics in 1856, this time as a member of the newly formed Republican Party. From 1859 to 1861, he served as Ohio’s governor and raised the state’s requisite number of troops for the Civil War, aiding those loyal to the Union and living in western Virginia in action against the Confederates.

Though Dennison failed to secure a Republican renomination for governor, President Abraham Lincoln tapped him in 1864 to serve as chairman of the Republican National Committee. Later that year, Lincoln called on Dennison again, this time to serve as postmaster general. Dennison agreed and remained in the position throughout the Lincoln administration and into the first year of the administration of President Andrew Johnson.

After resigning his office in 1866, William Dennison Jr. remained active in civic and business affairs in Ohio until his death in 1882.