Francis M. Guernsey was born on February 22, 1839, in Guilford, New York, to Jonathan Guernsey and Frances Putnam. His father was a physician who owned $5,000 of real estate in 1850. Guernsey grew up and attended school in Guilford before enrolling at Oberlin College in the late 1850s. He graduated around 1860 and moved to La Port, Indiana, where he established a commercial school. By the fall of 1861, he was living in Berlin, Wisconsin.
In June 1862, he celebrated "our glorious old Flag" and hoped for the "speedy termination of this fratracidal war." He enrolled in the Union army on August 6, 1862, and mustered in as a private in Company C of the 32nd Wisconsin Infantry later that day. He was promoted to sergeant major on September 25, 1862, to 1st lieutenant on May 3, 1864, and finally to captain on April 6, 1865. The regiment took part in the Vicksburg campaign, the Atlanta campaign, the March to the Sea, and the Carolinas campaign. In June 1864, he celebrated news of the National Union Party's national convention, which nominated Abraham Lincoln for president and called for emancipation and the "complete suppression of the Rebellion." As Guernsey explained, the convention "gives me more faith in our ability to conquer than a fresh Army of Five hundred thousand men. it shows that the heart of the people of the mighty north is in the right place...that rather than retrace the steps already taken by the Government they will fight it out to the last man and the last dollar." He mustered out in Crystal Springs, Maryland, on June 12, 1865.
Guernsey settled in Almond, Wisconsin, after the war, and he married Frances Eugenia Doty on August 29, 1865. Their daughter Ella was born around 1868. The family moved to Clintonville, Wisconsin, in 1867. He was appointed postmaster of Clintonville in April 1868, and he later served as county commissioner and a member of the county board.
By 1870, he was working as a “dealer in dry goods,” and he owned $1,500 of real estate and $1,500 of personal property. A decade later, he was working as a lawyer. He joined the Republican Party, and he secured a seat in the Wisconsin legislature in 1878. He applied for a federal pension in June 1880, complaining of a hernia. He moved to Santa Barbara, California, by the early 1910s, and he died there on June 24, 1919.