During his time as colonel, he assisted the Union Army in capturing forts during the Siege of Petersburg, and was discharged in June 1865 after suffering a gunshot wound.

Winslow was brevetted brigadier general by president Abraham Lincoln on April 9, 1865, for "brave and gallant conduct" during the siege.

After the war, he returned to politics, being re-elected as district attorney in 1865 and serving as a delegate and chairperson in the 1908 Republican National Convention.

[1][10] Upon the outbreak of the American Civil War, Winslow resigned as district attorney and volunteered to become a First lieutenant in the Black River Corps, a militia unit in Watertown, on May 13, 1861.

[8][11] While fighting in the Northern Virginia campaign, Winslow contracted typhoid fever, and he resigned from the Union Army on December 18, 1862, and received an honorable discharge.

[8][16] After the Battle of Five Forks on April 1, 1865, General Ulysses S. Grant commanded the Union Army to attack Petersburg, Virginia.

[18] Winslow later remembered that while getting ready to capture the fort, he heard "Screaming, hissing shot and shell, interspersed with the sharp whiz and ping of leaden bullets, seemed passing everywhere above our heads".

[22] The Confederate Army still maintained a secondary line and still held Fort Mahone, and they shot at Winslow and his regiment.

He was shot below between his lower right ribs by a Minié ball, which passed through his body and came out on the left side near his spine.

[11] My dear colonel, It is with sincere pleasure that I inform you that I have recommended your promotion to the rank of Brigadier General by brevet for bravery and gallant conduct on the field at the assault on the enemy's lines in front of Petersburg, April 2, 1865 [...] I am very happy, Colonel, to make this acknowledgment of your meritorious services as commander of your regiment, and of the gallant and judicious manner in which you handled your regiment in my presence during the engagement of the 2d of April, an engagement that will be forever memorable in our nation's history.

[8][27] In June 1908, Winslow was a delegate in the 1908 Republican National Convention to nominate a candidate for member of the 28th congressional district of the United States Congress, being the unanimous vote to be the chairperson.

[33] He was a member of the Grand Army of the Republic until his death, and he helped start the Joe Spratt Post Number 323 in Watertown.