According to L. Sprague de Camp, Pratt was born near Tonawanda , New York, and attended Hobart College for one year. During the 1920s he worked for the Buffalo Courier-Express and on a Staten Island newspaper. In 1926, he married Inga Stephens Pratt, an artist. In the late 1920s he began selling stories to pulp magazines. Again, according to de Camp's memoir, when a fire gutted his apartment in the 1930s he used the insurance money to study at the Sorbonne for a year. After that he began writing histories.

Wargamers know Pratt as the inventor of a set of rules for civilian naval wargaming before the Second World War. This was known as the "Naval War Game" and was based on a wargame developed by Fred T. Jane involving dozens of tiny wooden ships, built on a scale of one inch to 50 feet. These were spread over the floor of Pratt's apartment and their maneuvers were calculated via a complex mathematical formula. Noted author and artist Jack Coggins was a frequent participant in Pratt's Navy Game, and De Camp met him through his wargaming group.

Pratt established the literary dining club known as the Trap Door Spiders in 1944. The name is a reference to the exclusive habits of the trapdoor spider, which when it enters its burrow pulls the hatch shut behind it. The club was later fictionalized as the Black Widowers in a series of mystery stories by Isaac Asimov. Pratt himself was fictionalized in one story, "To the Barest", as the Widowers’ founder, Ralph Ottur.

He was also a charter member of The Civil War Round Table of New York, organized in 1951, and served as its president from 1953-1954. In 1956, after his death, the Round Table's board of directors established the Fletcher Pratt Award in his honor, which is presented every May to the author or editor of the best non-fiction book on the Civil War published during the preceding calendar year.

Aside from his historical writings, Pratt is best known for his fantasy collaborations with de Camp, the most famous of which is the humorous Harold Shea series, was eventually published in full as The Complete Compleat Enchanter (1989, ISBN 0-671-69809-5). His solo fantasy novels The Well of the Unicorn and The Blue Star are also highly regarded.

Pratt wrote in a markedly identifiable prose style, reminiscent of the style of Bernard DeVoto. One of his books is dedicated "To Benny DeVoto, who taught me to write."

Several of Pratt's books were illustrated by Inga Stephens Pratt, his wife.

Novels

• Alien Planet (1932)
• Land of Unreason (1941) with L. Sprague de Camp
• The Well of the Unicorn (1948)
• The Carnelian Cube (1948) with L. Sprague de Camp
• The Blue Star (1952)
• Double Jeopardy (1952)
• The Undying Fire (1953) (originally titled: The Conditioned Captain)
• Invaders from Rigel (1960)

Series

Harold Shea

• The Enchanter Stories of L. Sprague de Camp and Fletcher Pratt (2007) with L. Sprague de Camp
• The Complete Compleat Enchanter (1989) with L. Sprague de Camp
• The Compleat Enchanter (1975) with L. Sprague de Camp
• * The Incomplete Enchanter (1940) with L. Sprague de Camp
• * The Castle of Iron (1941) with L. Sprague de Camp
• Wall of Serpents [vt The Enchanter Completed (1980 UK)] (1960) with L. Sprague de Camp

Collections

• Double in Space (1951)
• Double Jeopardy (1952)
• Tales from Gavagan's Bar (1953, expanded 1978) with L. Sprague de Camp

Anthologies

• World of Wonder (1951)

Twayne Triplet

• 1 The Petrified Planet (1952)
• 2 Witches Three (1952)

Nonfiction

• Fletcher Pratt's Naval War Game (1940)
• A Man and His Meals (1947)
• World of Wonder : an Introduction to Imaginative Literature (1951)

Science

• All About Famous Inventors and Their Inventions (1955)
• All About Rockets and Jets (1955) illustrated by Jack Coggins
• Rockets, Jets, Guided Missiles and Spaceships (1951) with Jack Coggins
• By Space Ship to the Moon (1952) with Jack Coggins
• Rockets, Satellites and Space Travel (1958) with Jack Coggins

History and Biography

Naval History

• The Compact History of the United States Navy (1957)
• Empire and the Sea (1946) with Inga Stephens
• Fighting Ships of the U.S. Navy (1941) illustrated by Jack Coggins
• Fleet Against Japan (1946)
• The Navy has Wings; the United States Naval Aviation (1943)
• The Navy, a History; the Story of a Service in Action (1938)
• The Navy's War (1944)
• Night Work: the Story of Task force 39 (1946)
• Preble's Boys; Commodore Preble and the Birth of American Sea Power (1950)
• Sea Power and Today's War (1939)
• Ships, Men - and Bases (1941) with Frank Knox
• A Short History of the Army and Navy (1944)

The Napoleonic Wars

• The Empire and the Glory; Napoleon Bonaparte: 1800-1806 (1948)
• The Heroic Years; Fourteen Years of the Republic, 1801-1815 (1934)
• Road to Empire; the Life and Times of Bonaparte, the General (1939)

The Civil War

• The Civil War (1955)
• Civil War in Pictures (1955)
• Civil War on Western Waters (1956)
• The Military Genius of Abraham Lincoln : an Essay (1951) with Colin R. Ballard
• The Monitor and the Merrimac (1951)
• Ordeal by Fire; an Informal History of the Civil War (1935)
• Stanton, Lincoln's Secretary of War (1953)

World War II

• America and Total War (1941)
• The Marines' War, an Account of the Struggle for the Pacific from Both American and Japanese Sources (1948)
• The U.S. Army : a Guide to its Men and Equipment (1942) with David Pattee
• War for the World; a Chronicle of Our Fighting Forces in World War II (1950)
• What the Citizen Should Know about Modern War (1942)

Other

• The Battles that Changed History (1956) ISBN 0-486-41129-X
• The Cunning Mulatto and Other Cases of Ellis Parker, American Detective (1935) with Ellis Parker
• Eleven Generals; Studies in American Command (1949)
• Hail, Caesar! (1936)
• The Lost Battalion (1938) with Thomas M. Johnson
• Muscle-power Artillery (1938)
• My Life to the Destroyers (1944) with L. A. Abercrombie
• Secret and Urgent; the Story of Codes and Ciphers (1939)
• The Third King (1950)