As an author and historian he later wrote and published several works about and involving the Civil War and how he viewed the cause of both the North and South.

[5][6] Before the American Revolutionary War Hosmer's great-grandfather was a member of the Massachusetts Senate, when Samuel Adams was presiding officer.

[4][9] The Hosmer family can trace their roots back to a small community of Ticehurst, in the county of Sussex, England, in the sixteenth century[1][10] Hossmer was ordained a minister in 1861, and, like his father, became the pastor for the Unitarian congregation in Deerfield, Massachusetts.

[9] Feeling bound by duty he later left the ministry and joined the Union Army to serve in the American Civil War.

[15] While waiting to be deployed at the Siege he volunteered his medical skills treating the wounded at the army hospital for a week's duration.

[17][19] Throughout his term of service he wrote and prepared notes and letters, in the form of a diary, chronicling his experiences and observations, many involving Major-General Banks.

At the time he had no intention of publishing them later, committing that effort explicitly for the interest of his father and family and various close friends, which they eventually would receive.

At the urging of friends and relatives these accounts were organized and edited and were later published in a book in 1864, entitled, The Color Guard, being a corporal's notes of military service in the Nineteenth Army Corps, which outlined Hosmer's experiences during the war.

His work received good reviews from many well known literary critics of the time and was widely read in both America and England.

The Boston Daily Advertiser said Hosmer's work "is written in a delightfully graphic epistolary style, and is really one of the most sterling books that the war has called into existence."

The New York Evening Post said, "The prevailing tone of the book, however, is cheerful, hopeful, candid, and altogether Christian.

(See listing in Further reading) In 1893 Doctor Elliott Coues published an account of the expedition based on Biddle's 1814 publication but it became very scarce and out of print.

Hosmer, familiar in the affairs of Western history, undertook the task of supervising the reprinting of the complete work of 1814 and added a table of contents, comprehensive nineteen page introduction,[26] and an analytic index.

[12] During his career Hosmer corresponded with a variety of prominent literary men and educators of the late 19th century, including Ralph Waldo Emerson, Henry Adams, George Bancroft, Charles Eliot Norton, Oliver Wendell Holmes,[e] Edward Everett Hale, and John Fiske.

In the final years of Hosmer's life he wrote an extensive two volume auto-biography, which is housed at the Minnesota Historical Society.