Thomas Welsh was born on May 5, 1824, in Columbia, Pennsylvania. He was educated in the town's public schools and soon became interested in the lumber business of which Columbia Was then an important center. He entered the Mexican War as a private in the 2nd Kentucky Regiment, was promoted to first sergeant, was severely wounded at Buena Vista while serving under Zachary Taylor, subsequently took part in Winfield Scott's campaign against the city of Mexico as a second lieutenant of the 11th U. S. Infantry, and was discharged in 1848. In the years before the Civil War, Welsh had prospered as a merchant, canal boat owner, justice of the peace, and lock superintendent at Columbia. He recruited a company of the 11th Pennsylvania (a ninety-day command) in a matter of hours one evening in April, 1861, became its lieutenant colonel, and, after its muster out in July, was appointed colonel of the 45th Pennsylvania on October 21, 1861. He went to Charleston Harbor in 1862, commanding a brigade of H. G. Wright's division there in June, but he soon was returned to the Army of the Potomac, and at South Mountain and Sharpsburg Welsh commanded the 2nd Brigade of Orlando B. Willcox's division of Ambrose E. Burnside's IX Corps (the brigade had little to do with the storming of "Burnside's Bridge" over Antietam Creek, however). At Fredericksburg, Welsh was again in command of the 45th Pennsylvania, despite the fact that he had been promoted brigadier general of volunteers on November 29, 1862. He went to Cincinnati the following April for assignment by Burnside, who was then commanding in the Department of the Ohio, but in the meantime Welsh's appointment as brigadier had expired for want of Senate confirmation. He was reappointed, however, on March 23 to rank from March 13, 1863, and appears to have been duly confirmed. In the operations incidental to Burnside's defense of his far-flung department, Welsh was assigned to command of the 1st Division of the IX Corps, but in the course of the Vicksburg campaign contracted some form of malarial fever which caused his death in Cincinnati on August 14, 1863. He was buried in Mount Bethel Cemetery, Columbia.
Reference: Generals in Blue. Lives of the Union Commanders by Ezra J. Warner. Louisiana State University Press. Baton Rouge.