Michael H. Leary, a union soldier, was employed as a printer in Boston, Massachusetts prior to enlisting with the Irish-American dominated Ninth Massachusetts Regiment on June 11, 1861. The Ninth Massachusetts Regiment spent the months of May and June at Camp Wightman on Long Island in Boston Harbor. Encamped at Emmart's Farm until after the Battle of Bull Run, the Ninth Regiment moved to Arlington Heights, Virgina for a brief time. The winter was spent at Miners Hill, Virginia until March 10, 1862. Serving alongside the Fifth Corps during the Peninsular Campaign, the Ninth Regiment was engaged at Hanover Court House, Mechanicsville, Gaines Mill, and Malvern Hill in Virginia. During the Second Battle of Bull Run, Antietam, and Fredericksburg, the Ninth Regiment remained in reserve. The winter of 1862-1863 was spent at Falmouth, Virginia.

Leary deserted the Ninth Massachusetts Regiment on January 28, 1863, but returned and was eventually transferred to the Thirty-Second Massachusetts Veteran Volunteer Regiment when his three-year term with the Ninth expired. Daniel G. McNamara, a former Lieutenant with the Ninth Regiment and military historian, details the furloughs granted to the soldiers of the Ninth Regiment during the month of February. McNamara recalls that some men overextended their stays at home, "With the exception of a few desertions and 'stay-overs' the honorable action of the men was an example that would put to shame the above mentioned commissioned officers (Daniel G. McNamara, 277)." Leary was most likely one of these stragglers. His letters indicate a great desire to be with Nellie, so he may have remained at home in Boston longer than allowed.

In 1863, the Ninth Regiment was engaged in Pennsylvania at Gettysburg as well as at Chancellorsville in Virginia. After spending the winter in quarters at Bealton Station, Virginia, the regiment participated in the Wilderness Campaign in May 1864. For the 170 men who re-enlisted from the Ninth Regiment, as well as other Massachusetts Regiments, the Wilderness Campaign continued with action at Spotsylvania, North Anna and Bethesda Church in Virginia. The Thirty-Second Regiment participated in the Siege of Petersburg from June 1864 through April 1865 and was part of the pursuit of Lee's Army of Northern Virginia at Appomattox. Assigned to Company H as a Private, Leary was mustered out of the army on June 28, 1865, receiving his final discharge on July 11.

Sources: The History of the Ninth Regiment, Daniel G. McNamara (Boston: EB Stillings & Co., 1899) The Irish Ninth in Bivouac and Battle, M.H. McNamara (Boston: Lee & Shepard, 1867) ‘The Fighting Ninth’ for Fifty Years and the Semi-Centennial Celebration, Frank J. Flynn (Boston: s.n. 1911?) Commanding Boston’s Irish Ninth: The Civil War Letters of Colonel Patrick R. Guiney, 9th Massachusetts Volunteer Infantry, Christian G. Samito Ed. (New York: Fordham University Press, 1998)