Colonel Henry A. Weeks received, on the return of the 12th Militia from its three months' service, authority to reorganize it for a service of three years. While the regiment as a body did not re-enter the service, a majority of the officers and enlisted men of the new organization, which was named - 12th Militia - were from the original regiment. The men were recruited principally in New York city, Patchogue and Buffalo, and mustered in the service of the United States for three years in October, November and December, 1861, and January, 1862. In January, 1862, Company A was transferred to the 102nd N.Y. Volunteers, and the seven remaining companies of the regiment consolidated into five, B, C, D, E, and F; Company G being merged into Company C, and K into E. These five companies left the State February 5, 1862.

February 8, 1862, the companies of the 12th Militia joined the battalion of five companies of the 12th N.Y. Volunteers, and completed the regimental reorganization of the latter, all the companies retaining their alphabetical designation. May 1, 1863, the two years' men were returned to Elmira and their alphabetical designation. May 1, 1863, the two years' men were returned to Elmira and there honorably discharged and mustered out May 17, 1 863; the a three years' men were consolidated into a battalion of two companies, D and F, and this battalion, commanded by Maj. Henry W. Ryder, was, June 2, `1864, transferred to the 5th Veteran N.Y. Volunteers, as Companies F and E, respectively, of the latter.

The regiment served at Washington, D.C., from May 31, 1861; in the 4th Brigade, 1st Division, Army of Northeastern Virginia, from July, 1861; in Richardson's Brigade, Division Potomac, from August 4, 1861; in Wadsworth's Brigade, McDowell's Division, Army of the Potomac, from October 15, 1861; in Butterfield's Brigade, Porter's DIvision, 3rd Corps, Army of the Potomac, from March 13, 1862; in the 3rd Brigade, 1st Division, 5th Corps, Army of the Potomac, from May, 1862; the battalion as Provost Guard, 5th Corps, from May 1863.

During its service the regiment lost

-by death, killed in action, 3 officers, 44 enlisted men;

-of wounds received in action, 22 enlisted men;

-of disease and other causes, 1 officer, 67 enlisted men;

-total, 4 officers, 133 enlisted men;

-aggregate, 137; of whom 4 enlisted men died in the hands of the enemy; and it took part in the following engagements: