He joined the Union Army as a private in Company E of the 49th New York Volunteer Infantry in June, 1861, rising to orderly sergeant.
With the Army of the Potomac his unit participated in battles which included Young's Mill, Yorktown, Williamsburg, Mechanicsville, Fair Oaks, Gaines's Mill, Savage's Station, Malvern Hill, Second Bull Run, South Mountain, Antietam, Fredericksburg, Williamsport, Gettysburg, Mine Run, Wilderness, Cold Harbor, Petersburg and others.
Walsh was assigned to the joint committee on printing[3] He was re-elected in 1884 as a Democrat, winning 1,012 votes to 543 for Republican James McManus and 5 for Prohibitionist J.
Fellow Trades' Assembly Assemblyman Daniel Hooker was re-elected that same year as a "Democratic Trades' Assembly" candidate, indicating some kind of tacit arrangement in those districts, although a Union Labor candidate ran for Congress in Milwaukee against Democrat Peter V. Deuster that year.)
[4] During his time in the Assembly, Walsh introduced a bill which would require all prison-made goods to be plainly labeled, and another abolishing the leasing of prisoners as contract labor.