During the American Civil War, Ashley took an active role in supporting the recruitment of troops for the Union Army. He also became a leader among the Radical Republicans, writing a bill to abolish slavery in the District of Columbia in 1862. In 1863 he introduced the first bill which ultimately (with Ashley as House Majority floor manager) became the Thirteenth Amendment to the United States Constitution by a 2/3 margin of 2 votes on January 31, 1865, formally abolishing slavery.