From 1849 to 1853, Devens was United States Marshal for Massachusetts, in which capacity he was called upon in 1851 to remand the fugitive slave, Thomas Sims, to slavery.

This he felt constrained to do, much against his personal desire; subsequently, he attempted in vain to purchase Sims's freedom, and many years later appointed him to a position in the United States Department of Justice in Washington, D.C.[1] Devens practiced law at Worcester, Massachusetts, from 1853 until 1861.

On April 16, 1861, Devens gave an impassioned speech at Mechanics Hall in Worcester in support of Lincoln's call for 75,000 volunteers.

Devens distinguished himself at the Battle of Cold Harbor, while commanding the 3rd Division/XVIII Corps in Ulysses S. Grant's Overland Campaign.

[4] Devens remained in the army for a year as commander of the military district of Charleston, South Carolina,[1] before mustering out and returning home.

He later served as the fifth Commander-in-Chief of the Grand Army of the Republic from 1873 to 1875 and was also a veteran companion of the Military Order of the Loyal Legion of the United States.

Devens was also a key figure in the investigation into the unlawful execution of Confederate veteran Calvin Crozier by soldiers of the 33rd Regiment, U.S.

[1] Devens was a close friend to President Hayes and his family, and was a favorite of Washington society during his time in office.