Born in Minneapolis, Minnesota, Aasen's mother, Kristi (Danielsen) from Rollag in Numedal, was an extremely tall Norwegian woman of around 2.20 m (almost 7 ft 3 inches) in height (latest information from September 2008, sets her height to 188 cm, almost 6 ft 2 inches). It is not certain who his father was, but according to Aasen's sister Evelyn (who died in 1988), his father was Alfred Aasen. When Aasen was ten years old, he and his mother moved from Ridgeway, Iowa (where his uncle Sam/Sevre lived with his wife Cornelia) to Sheyenne, North Dakota with his two younger siblings. Aasen was a Freemason. He raised to the degree of Master Mason at Highland Park Lodge No. 382, Los Angeles on July 14, 1924.
When in Sheyenne, Aasen's mother operated a restaurant. He attended school and helped out in the family business. In 1902, Aasen's mother died. He was taken into many homes and families. When a family he was staying with started to operate a hotel in Leeds, North Dakota, he moved with them there. Aasen's growth started slowly. When he was confirmed in the Lutheran faith in Grandfield Lutheran Church near Sheyenne, North Dakota, he was the shortest in his class. According to some sources, Aasen was around 2.74 m or 8 feet, 11½ inches (which, if true, would make him even taller than Robert Wadlow, the tallest verified person in history). The Top 10 of Everything 2010 edition states his height at 8 feet, 9.7 inches.
According to the 1978 edition of Guinness World Records he was only 7 feet (213.4 cm). Just before his death, at age 46, he was medically measured at 7 feet 0.9 inches, however he had lost some height due to age and could not stand completely straight anymore. In June 2008, Loma Linda University confirmed that the 7-foot-2.4-inch (219 cm) skeleton they had in their collection was John Aasen.
In 1922, Aasen was offered an acting job in a film called Why Worry? (produced by Hal Roach and released in 1923) alongside the comedian Harold Lloyd. Later, he acted in several other films like Bengal Tiger, Charlie Chan at the Circus, Growing Pains, Should Married Men Go Home?, Legionnaires in Paris, Two Flaming Youths, The Sting of Stings, Long Fliv the King and the Tod Browning film Freaks, in a small uncredited cameo appearance.
Aasen died from pneumonia on August 1, 1938 at Mendocino State Hospital in Mendocino, California. His body was later shipped to a doctor in Missouri for study and dissection. The skeleton was kept by the doctor, and eventually shipped to Loma Linda, California. Aasen's cremated soft parts were given a Masonic funeral at Forest Lawn Cemetery in Glendale, California.