Arnold grew up in Guilford, Connecticut. At age 11 he started working out after his father gave him a set of weights. During the late 1970s and early 1980s, despite following protein diets, he grew frustrated with his inability to put on much muscle mass. According to his version of events, Arnold's first contact with steroids happened when "a guy in a gym got him a cheap counterfeit steroid that contained just enough methyltestosterone that it added 10 pounds of muscle in all the right places." This sparked his interest in chemistry, and in 1990 Arnold graduated with a bachelor's degree in chemistry from the University of New Haven.
After graduation Arnold took a lab job in New Jersey that allowed him enough free time to research performance enhancers. He also took classes on organic synthesis at the University of Connecticut and Montclair State, and "devoured" books on supplements and steroids, studying both approved and unapproved Western drugs and those used by the East Germans in their doping heyday.
In 1996 he befriended Dan Duchaine, who introduced Arnold to Stan Antosh, the owner of Osmo Therapy, a supplement company then based in San Francisco. Antosh persuaded Arnold to move his research to a small company in Seymour, Illinois, called Bar North America, which was owned by Ramlakhan Boodram. In Seymour, Arnold reviewed old patents looking for drugs that had never made it to market or were used only briefly. Later that year he introduced androstenedione to the North American market, which became successful after Mark McGwire was found using it. But because their company didn't sell andro directly to consumers — but only as an ingredient to other supplement makers — Arnold missed out on a financial windfall.