De Forest was born in Seymour, Connecticut,, the son of a prosperous cotton manufacturer. He did not attend college, but instead pursued independent studies, mainly abroad, where he was a student in Latin, and became a fluent speaker of French, Italian, and Spanish. While yet a youth, he spent four years traveling in Europe, and two years in the Levant, residing chiefly in Syria. In 1850, he again visited Europe, making extensive tours through Great Britain, France, Italy, Germany, Greece, and Asia Minor. From that time, he wrote short stories for periodicals, having already authored several books. One of his earliest works, The History of the Indians of Connecticut, from the Earliest known Period to 1850, shows his interest in history. Written from 1847 to 1850, The History of the Indians of Connecticut is critical of the settlers treatment of the Pequots and of King Philip's War, which is somewhat surprising given the early date of the scholarship. The non-fictional work also foreshadows De Forest's later fiction in its subject, realism, and occasional violence. The honorary degree of A. M. was conferred upon him by Amherst College in 1859.
Civil War
With the advent of the American Civil War, De Forest returned to the United States. As a captain in the Union Army, he organized a company from New Haven, the 12th Connecticut Volunteers. He served constantly in the field until January 1865, taking an active part under Maj. Gen. Godfrey Weitzel's command in the southwestern states, and under Philip Sheridan in the Shenandoah Valley. Graphic descriptions of battle scenes in Louisiana, and of Sheridan's battles in the valley of the Shenandoah, were published in Harper's Monthly during the war by Major De Forest, who was present on all the occasions thus mentioned, and though experiencing forty-six days under fire, received but one trifling wound. De Forest mustered out from the volunteer army in 1865 with the brevet rank of major.
Postbellum
After being mustered out of the army with the rest of the Veteran Reserve Corps of which he was the adjutant general, De Forest transferred to the Bureau of Refugees, Freedmen and Abandoned Lands shed light on the conditions in the South during the Reconstruction. His magazine articles of his time in the army were also collected published posthumously as A Volunteer's Adventures. In 1867, De Forest published his most significant novel, Miss Ravenel's Conversion from Secession to Loyalty. William Dean Howells praised him as a "realist before realism was named," but most early critics argued that the Romantic elements of De Forest's plot mixed poorly with the admirable realism of the battle scenes, and the novel fell through with the audience in 1867. Reeditions in 1939 and 1956 reintroduced De Forest as an author, but the full range of his experimentalísm in this early novel has still not been fully understood. In Miss Ravenel's Conversion, De Forest tried to come to grips with writing experiences De Forest himself had, and which did not fit any of the idealist and romantic patterns that war literature had followed so far. Consequently, there are a number of scenes that portray war with a graphic sense of bloody reality, but there are also burlesque and comical passages, as well as reflective moments. Writing for The Nation a year later, De Forest called for a more general movement in American literature toward realism; the essay's title, "The Great American Novel," is generally credited as being the first known use of the term. He died in New Haven, Connecticut, of heart disease.
Writing
De Forest wrote essays, a few poems, and about fifty short stories, numerous military sketches, and book reviews, most of which were anonymous. In 1873, he contributed to The Atlantic Monthly a short serial story entitled "The Lauson Tragedy." His published books include:
The History of the Indians of Connecticut, from the Earliest known Period to 1850
Oriental Acquaintance, a sketch of travels in Asia Minor
Witching Times
European Acquaintance
Seacliff, a novel
Miss Ravenel's Conversion from Secession to Loyalty