Isaac Plumb, Jr. (1842-1866) served the Union Army during the Civil War from his enlistment in the fall of 1861 until his death at the Battle of Cold Harbor in 1864, and saw action at Fair Oaks, the Seven Days Battle, Antietam, Fredericksburg, Chancellorsville, Gettysburg, the Wilderness, and Cold Harbor. During his experience as a Union soldier, Plumb's attitudes towards politics, the War, the North and South, and slavery underwent enormous transformations, from wild patriotism, to bitterness, and finally to a more realistic patriotism.

Plumb, the son of Isaac Plumb (1808-1891), a furniture dealer, and Catherine Eliza Grant (1822-1914), was raised in Sherburne, New York, in Chenango County along with his siblings Anna Plumb (1845-1917) and Henry Grant Plumb (1847-1936). After earning his education at Sherburne Academy, Isaac Plumb Jr. began working for Leroy Fairchild's Gold Pen Manufactory. In the autumn of 1861, at the age of 19, Plumb enlisted in the Union Army, mustered in as a sergeant, and was assigned to the 61st New York Infantry Regiment. Over his three years of service, Plumb rose in rank to captain, serving under Francis C. Barlow (1834-1896) and Nelson Appleton Miles (1839-1925). Plumb was initially enthusiastic about the War, feeling that Union had to be preserved and that he wanted to "help give those southern fellows a lesson" (November 12, 1860); however, after his first major battle at Fair Oaks, Plumb quickly began to understand the realities of battle and his expectation of surviving the war diminished. Over the years, he slowly became disillusioned with Abraham Lincoln and the government, the condition of the Union Army, the quality of leadership, the Union's ability to easily defeat the Confederates, and the government's statement that the war was being fought to end slavery. On January 19, 1863, Plumb wrote, "it may sound very unpatriotic and unsoldier-like in me, but I must express my honest opinion. That we are whipped." By the end of 1863, though, Plumb seemed to have regained some confidence in the reasons for the war, determining that slavery was a curse to the country and needed to be ended and that "Abe Lincoln [was] a brick" (December 16, 1863). Throughout his entire army service, Plumb expressed regret and deep sadness about the loss of life and, following the Battle of Antietam, wrote, "as I looked upon the hundreds of the dead upon the battlefield after the battle, I thought of the fathers and mothers, sisters and brothers, and husbands, how they would feel and how much misery that one battle had caused" (September 23, 1862). Plumb died on July 4, 1864, at Campbell Hospital in Washington, D.C., of wounds received on June 11, 1864, at the Battle of Cold Harbor.

The Plumb and Grant families appear to have been very close. Isaac Plumb, Sr. was involved in politics; his brother, David Henry Plumb (1818-1889), was a minister and served in the Union Army in the 18th Army Corps; and his brother-in-law, Henry Grant (1820-1894), was a banker in New York City. During his service, Isaac Plumb Jr. corresponded frequently with his parents; his uncles David and Henry; his siblings Anna and Henry (a successful artist); and his cousin Mary Emily Folger and her husband, William J. Pell.