The regiment, under the command of Henry A. Barnum, left Syracuse on September 23, 1862, and within a short time joined General McClellan's army.

It was assigned to the Third Brigade, Geary's Division, Twelfth Corps, in which command it fought at Chancellorsville, losing there 15 killed, 68 wounded, and 103 captured or missing.

At Gettysburg the regiment participated in the famous defense of Culp's Hill, made by Greene's Brigade, in which the One Hundred and Forty-ninth, fighting behind breastworks, lost 6 killed, 46 wounded, and 3 missing, but inflicted many times that loss on its assailants.

With the Twelfth Corps, it was transferred to the Army of the Cumberland, and the Onondaga boys fought as bravely in Tennessee as in Virginia or at Gettysburg.

At Lookout Mountain, Tenn., they captured five flags while fighting under Hooker in that memorable affair, their casualties amounting to 10 killed and 64 wounded.