As a child, he was an avid reader of the numerous science fiction stories featuring Captain Proton, Flash Gordon and Buck Rogers. He also read the novel The War of the Worlds. (TOS - Strange New Worlds III short story: "The Aliens Are Coming!", TOS novel: From History's Shadow) He retained his interest in science fiction into his fifties. (TOS novel: From History's Shadow)
Wainwright fought in France during World War II. He and his wife Deborah were married shortly before he was shipped out to England in early 1944. Their only child, Michael, was born in the autumn of 1946. (TOS novel: From History's Shadow)
In July 1947, Captain Wainwright, then a member of the United States Army Air Corps, supervised the recovery of the Ferengi shuttle Quark's Treasure and had Quark, Rom and Nog incarcerated. Wainwright was suspicious of the three Ferengi, whom he consistently referred to as "Martians," believing that they were at the forefront of an invasion of Earth. Wainwright planned to use any means necessary to force them into revealing their plans but was stopped by Professor Jeffrey Carlson and his fiancée Nurse Faith Garland. After the Ferengi escaped, Wainwright was told by General Rex Denning that they had nothing to worry about as all they had ever found was a crashed weather balloon. (DS9 episode: "Little Green Men")
With the signing of the National Security Act into law by President Harry S. Truman in September of that year, he became a member of the United States Air Force, as did every other serving member of the Army Air Corps. On September 23, he was recruited into the newly created Majestic 12, a secret organisation whose mandate was to collect evidence of extraterrestial activity on Earth and develop strategies to combat potential alien invaders, by Professor Carlson. He was surprised by the offer as he was under the impression that Carlson considered him "to be little more than a typical brainwashed military robot, incapable of exercising any thoughts not already programmed into him by his superiors." He was also suspicious of Carlson and Garland's claim to have been manipulated by the Ferengi, using their "insidious mind-control powers," into helping them escape from Roswell. Carlson was aware of Wainwright's doubts about him but recruited him regardless.
On that occasion, Carlson told him that there was a theory that Orson Welles' War of the Worlds broadcast on October 30, 1938 - which had caused a mass panic in the United States - was used as a cover story for the discovery of an actual alien spaceship. However, Wainwright dismissed this theory, branding it "crazy." He noted that he had not heard the broadcast personally but read about it in the newspapers the following day.
Several months later, President Truman ordered the creation of another initiative, Project Sign, the primary mission of which was to investigate UFO sightings. Carlson had Wainwright designated as the liaison officer between the Majestic 12 and Project Sign. For almost a year, Wainwright and various other Air Force officers investigated reports submitted by both military personnel and civilians and were able to disprove the theory that the UFOs were secret planes of the Soviet Union. While they were unable to garner hard evidence of alien visitations, the witness reports and photographic evidence which they gathered were enough to convince senior government and military officials that there was extraterrestrial activity in the skies above not only the United States but all of Earth. However, over the course of 1948, the initial support which Project Sign enjoyed began to dissipate due to the lack of "tangible, actionable proof." A report was submitted to the Air Force chief of staff, General Hoyt Vandenberg, detailing the numerous incidents of alleged extraterrestrial activity. However, it was rejected. Project Sign was subsequently made public and was subjected to much ridicule. Consequently, it was disbanded.
It was replaced by Project Grudge. The initial mandate of this new organisation was to find plausible explanations for the UFO sightings other than extraterrestrials. Over time, its directive changed to ignoring and debunking any and all claims of extraterrestrial activity, becoming little more than a public relations body. Wainwright strongly considered resigning his commission on two separate occasions but Carlson talked him out of it. Following the mass sighting of numerous disc-like aircraft in the skies of New Jersey on September 10, 1951, Project Grudge's apathetic attitude towards UFOs was taken to task and it too was disbanded. The mandate of its replacement, Project Blue Book, was to vigorously pursue any information relating to UFOs and keep an open mind. Shortly thereafter, Wainwright was assigned as Project Blue Book's Air Force liaison officer at the Wright-Patterson Air Force Base in Dayton, Ohio with Airman First Class Allison Marshall, with whom he became close friends, as his assistant.
On April 21, 1952, Wainwright's wife Deborah left him after eight years of marriage, taking Michael with her, as she was tired of her husband placing his work ahead of her as well as his constant travelling. They divorced shortly afterwards and Deborah eventually remarried. Wainwright met her new husband on one occasion and considered him to be reliable and trustworthy, the sort of man whom Deborah and Michael needed in their lives.
The next day, Wainwright and Marshall travelled to Yuma, Arizona to investigate the latest UFO sighting and interview the witnesses, where they discovered an alien spacecraft at the Yuma Test Station which in the process of being reverse engineered by the facility's personnel. While investigating the ship, they were attacked by three agents of the Certoss, a faction of the Temporal Cold War from the 25th century, who were themselves attempting to gain access to the ship. One of their number, Etlun, was killed. Both her body and the spacecraft were sent to Professor Carlson for study.
By early 1955, Wainwright and Marshall were aware that Calvin Sutherland of the UFO magazine Watch the Skies was receiving information about Majestic 12 and Project Blue Book's investigations from a mole working within one or both of those organisations. On January 14 of that year, Wainwright broke into Sutherland's office in Los Angeles, California and, along a note which he hoped the journalist would believe was from his informant, provided him with false information regarding fictional UFO investigations. Wainwright hoped that publishing this false information would serve to discredit both Sutherland and Watch the Skies. This ploy worked for a time but Sutherland realised that he had been tricked after failing to find corroborating evidence of various alleged sightings.
On November 10, 1957, Wainwright - by then a major - and Marshall travelled to the mining town of Carbon Creek, Pennsylvania where they discovered a Vulcan science vessel which had crash-landed in the vicinity five weeks earlier. They intended to confiscate the ship and have it sent to Carlson for study. However, the Aegis agent Cynthia Foster used her servo to render them unconscious and erase their memories of finding the Vulcan ship as, from their Aegis benefactors, Foster and her partner Ian Pendleton were aware that Human-Vulcan first contact would not occur for over a century.
Soon after leaving Carbon Creek, Wainwright and Marshall began a romantic relationship. On July 29, 1958, they were contacted by Mestral, one of the survivors of the crashed Vulcan ship who decided to remain on Earth. Mestral revealed his alien nature and what he had learned of the threat posed by the Certoss to the two Air Force officers. He was able to prove his credentials by mind melding with Wainwright. Realising that Mestral could prove a useful and powerful ally, Wainwright recruited him into Majestic 12. However, he and Marshall elected against telling anyone other than Carlson that Mestral was an alien.
In 1962, Wainwright retired from the Air Force as he was aware that his work with Project Blue Book was detrimental to his career advancement. However, he was still employed by the Department of Defense as a civilian.
On September 27, 1963, Wainwright, Marshall and Mestral met with Carlson and Major Lucas Fellini at the Wright-Patterson Air Force Base to discuss the Certoss attack on the facility the previous day. After Fellini's departure, Mestral informed the other three that the Certoss were hoping to gain access to an unmanned Vulcan probe which had crashed on Earth and had been taken to the base for study. On that occasion, Wainwright learned that Sutherland's informant was none other than Carlson himself. He explained to Wainwright that he began supplying information to the journalist as numerous articles in Watch the Skies had provided solid leads for Majestic 12 over the years. Carlson believed that Sutherland was a natural when it came to investigative work and that he had proven himself to be a useful ally.
By this time, Michael was 17 years old and was strongly considering following in his father's footsteps and joining the Air Force. Wainwright's ex-wife Deborah was strongly against the idea.
On July 17, 1967, Wainwright and Marshall informed their fellow Project Blue Book agent Colonel Stephen Olson that a shooting star which had crashed in the forest in the High Sierras six days earlier had disappeared. They had received reports from eleven different witnesses who saw the crash but several of them disagreed as to the probable location, resulting in a significant delay in pinpointing the crash site. From the metal fragments which they discovered at the site, however, they were able to determine that an aircraft or spacecraft had crashed. A clearly exasperated Wainwright informed Olson, whom he considered to be a "moron," that the search had taken as long as it did due to the rugged terrain of the High Sierras, which made it impossible to bring cars up the mountain range. The two men almost came to blows when Olson passed a lewd comment about Marshall, whom he was aware was in a relationship with Wainwright, but she managed to talk them out of it. (TOS novel: From History's Shadow)
Unbeknownst to the three agents, the ship was, in actuality, the 29th century timeship USS Aeon and it had been promptly recovered by a hippie named Henry Starling, who not only witnessed the crash but was in close proximity to the crash site. (VOY episode: "Future's End")
On July 10, 1969, Wainwright met with John Christopher after the Air Force captain saw a UFO - the USS Enterprise which had been thrown back in time from 2266 - over Omaha, Nebraska. (TOS short story: "The Aliens Are Coming!", TOS novel: From History's Shadow)