In the 1950s he became well-known as an engaged, knowledgeable UFO researcher, arguing that the U.S. government should conduct appropriate research in UFO matters, and should release all their UFO files. Jerome Clark writes that "Keyhoe was widely regarded as the leader in the field" of ufology in the 1950s and early-to-mid 1960s.
In 1922, his arm was injured during an airplane crash in Guam. During his long convalescense, Keyhoe began writing as a hobby. He eventually returned to active duty, but the injury gave Keyhoe persistent trouble, and, as a result, he retired from the Marines in 1923. He then worked for the Geodetic Survey and U.S. Department of Commerce.
In 1927, Keyhoe managed a very popular coast-to-coast tour by Charles Lindbergh. This led to Keyhoe's first book, 1928's Flying With Lindbergh. The book was a quick success, and led to a freelance writing career, with many of Keyhoe's articles and fictional stories (mostly related to aviation) appearing in a variety of leading publications.
During World War II, Keyhoe reenlisted in the armed forces, and served in the Naval Aviation Training Division. He was promoted to Major during the war.
Keyhoe wrote a number of air adventure stories, for Flying Aces and other magazines, and created two larger-than-life superheroes in this genre. The first of these was Captain Philip Strange, referred to as "the Brain Devil" and "the Phantom Ace of G.2.". Captain Strange was an American intelligence officer during World War I who was gifted with ESP and other mental powers. His existence has been perpetuated beyond Keyhoe's stories as a minor member of the Wold Newton universe.
Keyhoe's other "superpowered" flying ace was Richard Knight, a World War I veteran who was blinded in combat but gained a supernatural ability to see in the dark. Knight featured in a number of adventure stories set in the 1930s (when the stories were written).
Many of Keyhoe's stories for the pulps were science fiction or weird fantasy, or contained a significant measure of these elements –- a fact that was not lost on later critics of his UFO books.
After some investigation, Keyhoe became convinced that the flying saucers were real. As their forms, flight maneuvers, speeds and light technology was apparently far ahead of any nation's developments, Keyhoe became convinced that they must be the products of unearthly intelligences, and that the U.S. government was trying to suppress the whole truth about the subject. This conclusion was based especially on the response Keyhoe found when he quizzed various officials about flying saucers. He was told there was nothing to the subject, yet was simultaneously denied access to saucer-related documents.
Keyhoe's article "Flying Saucers Are Real" appeared in the January, 1950 issue of True (published December 26, 1949) and caused a sensation. Though such figures are always difficult to verify, Air Force Captain Edward J. Ruppelt, the first head of Project Blue Book, reported that "It is rumored among magazine publishers that Don Keyhoe's article in True was one of the most widely read and widely discussed magazine articles in history."
Capitalizing on the interest, Keyhoe expanded the article into a book, The Flying Saucers Are Real, (1950), which would sell over half a million copies in paperback. He argued that the Air Force knew that flying saucers were extraterrestrial, but downplayed the reports to avoid public panic. In Keyhoe's view, the aliens — wherever their origins or intentions — did not seem hostile, and had likely been surveiling the earth for two hundred years or more, though Keyhoe wrote that their "observation suddenly increased in 1947, following the series of A-bomb explosions in 1945." Dr. Michael D. Swords characterized the book as "a rather sensational but accurate account of the matter." (Swords, p. 100)
Keyhoe wrote several more books about UFOs. Flying Saucers From Outer Space (1953) is perhaps the most impressive, being largely based on interviews and official reports vetted by the Air Force. The book included a blurb by Albert M. Chop, the Air Force's press secretary in the Pentagon, who characterized Keyhoe as a "responsible, accurate reporter" and further expressed guarded approval for Keyhoe's arguments in favor of the extraterrestrial hypothesis. Such endorsements only cemented the belief, held by some observers, that the Air Force's mixed messages about UFOs were due to a cover up.
Carl Jung argued that Keyhoe's first two books were "based on official material and studiously avoid the wild speculations, naivete or prejudice of other [UFO] publications." (Jung, xiii)
Others have disagreed with Keyhoe's assessments. In his 1956 book, Edward J. Ruppelt wrote, "the Air Force wasn't trying to cover up", and declared that "The problem was tackled with organized confusion".
Ruppelt's book indicates that Ruppelt held some dim views of Keyhoe and his early writings; Ruppelt noted that while Keyhoe generally had his facts straight, his interpretation of the facts was another question entirely. He thought Keyhoe often sensationalized material and accused Keyhoe of "mind reading" what he and other officers were thinking. Yet Keyhoe cites conversations with Ruppelt in later books, suggesting that Ruppelt may have occasionally advised Keyhoe.
NICAP founder Thomas Townsend Brown was ousted as director in early 1957 after facing repeated charges of financial ineptitude. Keyhoe replaced him; he was only slightly better at managing NICAP's finances, and the group continued their efforts
With Keyhoe in the lead, NICAP pressed hard for Congressional hearings and investigation into UFOs. They scored some attention from the mass media, and the general public (NICAP's membership peaked at about 15,000 during the early and mid 1960s) but only very limited interest from government officials.
However, there was increasing criticism of the Air Force's Project Blue Book. Following a widely publicized wave of UFO reports in 1966, NICAP was among the chorus which called for an independent scientific investigation of UFOs. The Condon Committee was formed with this goal in mind, though it quickly became enmired in infighting and, later, controversy. Keyhoe publicized the so-called "Trick Memo", an embarrassing memorandum written by a Condon Committee coordinator which seemed to suggest that the ostensibly objective and neutral Committee had determined to pursue a debunking operation well before even beginning their studies.
On 8 March 1958 Keyhoe appeared on The Mike Wallace Interview on ABC and spoke about flying saucers, contactees and the details of the Armstrong Circle Theatre censorship, which he blamed on the Air Force rather than CBS.
Keyhoe wrote one more UFO book, 1973's Aliens From Space. It promotes "Operation Lure", a scheme to entice extraterrestrials to land.
Beyond this book, Keyhoe had little contact with ufology as he settled into retirement. (He did, however, speak at a few UFO conferences after his ouster from NICAP). He joined MUFON's board of directors in 1981, but his membership was essentially in name only due to declining health, and he had little to do with the organization.
Several of Keyhoe's books are now in the public domain and are available online.