Born in Burlington, Vermont, Doolittle attended school in Montreal, Quebec, and moved to New York City in 1847, finally settling in Hillsdale, Michigan, where he was a store clerk before the start of the Civil War. Commissioned as a First Lieutenant in the 4th Michigan Volunteer Infantry (June 20, 1861), he was promoted to Captain and a company command that August. Wounded at the Battle of Gaines Mill (June 28, 1862), during the Peninsula Campaign, he transferred to the 18th Michigan Infantry that August as its commander, leading the 18th in the Western Theatre for the war's duration with various assignments in Kentucky, Tennessee, Alabama, and Georgia.

His troops helped defend Athens, Alabama, against Confederate cavalry under Joseph Wheeler and Decatur, Alabama, against John Bell Hood.

 

On April 22, 1865, President Johnson appointed Doolittle brigadier general of volunteers to rank from January 27, 1865. President Lincoln had nominated Doolittle for the appointment on January 30, 1865 and the U.S. Senate confirmed the nomination on February 14, 1865 but Lincoln was unable to make the formal appointment before he was assassinated. Doolittle was mustered out of the volunteer service in November 1865. On February 24, 1866, President Johnson nominated Doolittle for appointment to the brevet grade of major general of volunteers. 

 

Doolittle settled in Toledo after the war where he served as a bank cashier at Merchant's National Bank.