Following the war, Farnham became general counsel for the Vermont Copper Company and continued to work as both lawyer and administrator of the VCC for the rest of his life. In addition, he held a number of local and state political offices culminating in his defeat of Democrat Edward J. Phelps for the governorship of Vermont in 1880. 

After completing a single popular term as governor, Farnham returned to his law practice. In 1889 he also became president of the newly-formed New England Company, a group of Northern investors interested in developing the coal and iron deposits of northwestern Georgia. The New England Company was never a success, and Farnham spent much of the last decade of the nineteenth century trying to save it and the VCC from bankruptcy. Badly injured in a fall in November 1898, Farnham recovered sufficiently to resume some of his work but never regained full health. Roswell Farnham died at his home in Bradford on January 5, 1903, at the age of seventy-five. 

Three of Farnham’s four children lived to adulthood: Charles Cyrus Farnham (1864–1937), Florence Mary Osgood (1866–1958), and William M. Farnham (1869–1927). His first child, Roswell Phelps Farnham Jr., died in infancy in 1861. Farnham was predeceased by a half-brother, Cyrus C. Farnham, in 1863.  

Topics in this diary include the curriculum, faculty, and student experience at UVM in the late 1840s; Burlington and neighboring towns in the late 1840s, UVM’s Lambda Iota fraternity, Zachary Taylor and the Whig Party, and teaching in Vermont and Canada in the mid-nineteenth century. Near the end of the diary are several essays written by Farnham during his senior year at UVM. Topics in these essays include religion, natural history, and King Lear.