Marshall S. Eastman was born around 1831 in Maine. He married Mary Abbott in the 1850s, and they had at least five children: Marianna, born around 1855; Sarah, born around 1856; Marcia, born around 1858; Rosa, born around 1864; and George, born around 1869. They lived in Effingham, New Hampshire, and Eastman worked as a farmer. By 1860, he owned $1,500 of real estate and $360 of personal property. He enlisted in the Union army on October 29, 1861, and he mustered in as a musician in the 11th Maine Infantry later that day. He was discharged for disability on March 26, 1862.
Eastman returned to New Hampshire after the war. He moved to South Berwick, Maine, in the late 1860s and began working in a cotton mill. By 1870, he owned $1,600 of real estate and $500 of personal property. He applied for a federal pension in September 1879 and eventually secured one. By 1880, they were living in Saco, Maine, and by 1890, they were living in Rollinsford, New Hampshire. He died there on July 7, 1890.