Armstrong was born in Pittsburgh, Pennsylvania. He started playing the guitar at age 11, and moved to New York in the early 1960s in order to work as a studio musician and guitar repairman. In 1965 he opened his own guitar repair shop, 'Dan Armstrong's Guitar Service', on West 48th Street. The building was razed in 1968 to make room for 30 Rockefeller Plaza, and Armstrong relocated his shop, renamed 'Dan Armstrong Guitars', to 500 Laguardia Place in Greenwich Village. In 1968 the Ampeg Company of Linden, New Jersey hired Armstrong as a consultant to improve their Grammer line of guitars. He designed a new line of guitars and basses that were constructed of clear Plexiglas. These guitars had interchangeable pickups designed by Bill Lawrence who shared the Greenwich Village shop with Armstrong, and eventually took it over when Armstrong moved to London. The guitars had long sustain caused by the solid Plexiglas body, though that material made for a heavy guitar—around 10 lbs. There was a reissue, made in Japan, in 1998, where the reissue was compared to the 1968 original, as being identical. A second reissue of the Dan Armstrong guitar was launched in 2006. Armstrong moved to London in the early 1970s where he developed a new line of electric instruments, amplifiers and effects boxes. The Dan Armstrong London instruments were made of solid Honduran mahogany with sliding low impedance pickups, available as a six string guitar, and short-scale and long-scale basses. Armstrong also marketed a line of tube guitar and bass amplifiers and effects boxes, the Blue Clipper, Yellow Humper, Red Ranger, Purple Peaker, Green Ringer and Orange Squeezer. In 1977 Armstrong and his wife, Vicki O'Casey, moved back to the United States. A licensing and manufacturing agreement was reached with Musitronics to re-release the effects boxes. Armstrong also developed a line of pickups for Schecter Guitar Research, a new amplifier for Fender. The couple returned to England, where they lived in Ashford, Kent, in the late 1990s, but again moved back to America after several years. After suffering from emphysema for many years, Armstrong died from a combination stroke and heart attack in Los Angeles on June 8, 2004. Armstrong effects boxes continue to be made under license from his son, Kent Armstrong, who is also a maker of guitar pickups.
had a personal relationship with Armstrong until around 1971. He is the subject of the song "Dan My Fling" from her 1971 debut album and has been reported by some to be the subject of her 1972 song "You're So Vain". In 2010, in relation to a suggestion that David Geffen was the subject of "You're So Vain", Simon stated that when she released the song she had not yet met Geffen.
