William Cosgrove was born in New York to Stephen and Hanna Cosgrove in March of 1841. Prior to 1860, the Cosgrove family moved to Milton, Ohio (Ashland County), where Stephen and William worked on the family farm. William was the first of nine children: Willis (born 1842), Malissa (born 1844), Mary (born 1846), Marvin [sp?] (born 1848), Henry (born 1850), Ellis (born 1852), Monroe (born 1854), and Marion (born 1857).

In September of 1861, William Cosgrove mustered into Battery D of the 1st Regiment of the Ohio Light Artillery. This battery saw its first major action when they participated in General Don Carlos Buell's campaign in Northern Alabama and Middle Tennessee between June and August of 1862. They then participated in a number of small campaigns in Kentucky and Tennessee before being assigned to the Middle Tennessee (or Tullahoma) Campaign from June 23 to July 7, 1863, and to the Knoxville Campaign from November 4 to December 23, 1863.

Shortly before writing the letter in this collection, Cosgrove transferred from Battery D to Battery G of the 1st Regiment of the Ohio Light Artillery. With this unit, he was stationed in Huntsville, Alabama, until March of 1865. Battery G then campaigned in East Tennessee from March 20 until April 5, 1865, when they were moved to Nashville. They moved again, this time to New Orleans, Louisiana, on June 16, 1865, and served there until they were ordered home for muster out on August 31, 1865.

After the war ended, Cosgrove returned to Ohio and settled in Holmes (Crawford County) where he worked as a mechanic and carpenter. He married Rebecca in approximately 1868, and they had five children, four of whom survived them. Rebecca died on November 23, 1916, and William followed on January 15, 1924.