Adamski emigrated with his family to New York City in 1893. He served with the U.S. Army from 1913-1916 and with the National Guard until 1919. His post-war activities are unclear but he settled in California in the late 1920s and by 1940 was a self-appointed professor of the Royal Order of Tibet, an organisation he founded. He gave lectures on philosophy and "Universal Law" but the organisation had a short existence.
In 1949 Adamski wrote a science fiction book which had a space travel theme. Called "Pioneers of Space: A Trip to the Moon, Mars and Venus" and published by Leonard-Freefield Co of Los Angeles, it was to greatly influence his subsequent writings.
Adamski began claiming sightings of UFOs and taking photographs and film of them, and in the early 1950s he was describing encounters with aliens. His best publicised claimed encounter was on November 20, 1952 when he and friends were in the Mojave Desert. Adamski said they saw a silver disc land and an entirely humanoid alien exit from the disc. Adamski said he approached him and they communicated telepathically. Adamski said the alien, named Orthon, was from Venus and expressed to Adamski his concern over the development of nuclear weapons. Adamski claimed to have met other aliens later (mostly from Venus but also from Mars and Saturn) and said he was taken on flights by them, including one around the moon where he observed valleys and cities.
Five years after publishing his first book, Adamski took some of the fictional material from that book and combined it as fact within "Flying Saucers Have Landed". He later published "Inside The Flying Saucers" and "Flying Saucer Farewell" and travelled the world on lecture tours.
He had considerable support from UFO proponents worldwide, but also attracted much scorn, TIME magazine going so far as to call him "a crackpot from California". The photographs taken by the first Soviet lunar probe in 1959 he denounced as fakes, and after he announced he was going to Saturn for a conference, many of his supporters became disaffected and his reputation rapidly declined. He died, aged 74, of a heart attack, in Maryland.