Falls is the eldest son of Sir Charles Falls, an Ulster landowner in County Tyrone. He receives his formal education at the Portora Royal School, Enniskillen, and the University of London. At the age of 27 he publishes his first book, Rudyard Kipling: A Critical Study (1915).
During World War I Falls receives a commission into the British Army as a subaltern in the Royal Inniskilling Fusiliers. He serves as a Staff Officer in the Head Quarters of the 36th (Ulster) Division and the 62nd (2nd West Riding) Division during the conflict. He receives the French Croix de guerre and is discharged from the British Armed Forces with the rank of Captain.
Immediately after leaving the British Army Falls writes a history of one of the Divisions that he had served with during the war, entitled The History of the 36th (Ulster) Division, which is published in 1922.
From 1923 to 1939 Falls is employed by the Historical Section of the United Kingdom Government‘s Committee of Imperial Defence, researching and writing the text of several volumes of the British Government’s official History of the Great War. He serves as the military correspondent for The Times during World War II from 1939 to 1945.
Falls holds the post of Chichele Professor of Military History at All Souls College, Oxford from 1946 to 1953. From the late 1940s through to the end of his life in the early 1970s he is a productive writer of military histories, publishing in-depth detailed studies as well as general works for the commercial market, his final two titles being published posthumously.
The historian Sir Michael Howard later describes Falls’ work The History of the 36th (Ulster) Division (1922) as “containing some of the finest descriptions of conditions on the Western Front to be found anywhere in the literature of the war.”