Sargent enlisted as a private in the 5th Vermont Infantry, Company D at Stowe, Vermont in August 1861, and was commissioned as the first lieutenant of Company K of that regiment in May 1865. Engagements during his tenure of service included: the defense of Washington, D.C., Siege of Yorktown (1862), Battle of Williamsburg, Battle of Garnett's and Golding's Farm, Battle of Savage's Station, Maryland Campaign, Battle of Crampton's Gap, Battle of Antietam, Battle of Fredericksburg, Battle of Salem Church, Battle of Gettysburg, Battle of the Wilderess, Battle of Spotsyvlania Court House, Battle of Cold Harbor, Battle of Cedar Creek, Third Battle of Petersburg, and the Battle of Appomattox Court House. He was awarded the U.S. Medal of Honor for his display of gallantry on April 2, 1865, during which he scaled, and placed the American flag on, the parapet of a Confederate fort at Petersburg, Virginia. That award was conferred in October 1891.