Born in New York City, Fixx was a graduate of Trinity School in New York and Oberlin College in Ohio. His father, Calvin Fixx, was an editor at Time who worked with Whittaker Chambers.  Fixx was a member of the high-IQ club, Mensa, and published three collections of puzzles: Games for the Super-Intelligent, More Games for the Super-Intelligent, and Solve It! The back flap of his first book says: “… He spent his time running on the roads and trails near his home, training for the Boston Marathon.”  Fixx started running in 1967 at age 35. He weighed 240 pounds (110 kg) and smoked two packs of cigarettes per day. Ten years later, when his book, The Complete Book of Running (which spent 11 weeks at No. 1 on the best-seller list) was published, he was 60 pounds (27 kg) lighter and smoke-free. In his books and on television talk shows, he extolled the benefits of physical exercise and how it considerably increased the average life expectancy.  The cover of his book The Complete Book of Running featured Fixx’s muscular legs against a red cover. The book sold over a million copies.  In 1980 Fixx wrote a follow up book titled Jim Fixx’s Second Book of Running: The Companion Volume to The Complete Book of Running.  In 1982 Fixx published Jackpot!, the story of what happened after the publication of The Complete Book of Running when he experienced the “Great American Fame Machine”, becoming richer and more celebrated than he could have imagined.  Maximum Sports Performance, published posthumously, discusses the physical and psychological benefits of running and other sports, including increased self-esteem, acquiring a “high” from running, and being able to cope better with pressure and tension.

On July 20, 1984, Fixx died at age 52 of a fulminant heart attack, after his daily run on Vermont Route 15 in Hardwick. The autopsy revealed that atherosclerosis had blocked one coronary artery 95%, a second 85%, and a third 70%. Still, medical opinion continued to uphold the link between exercise and longevity. In 1986 exercise physiologist Kenneth Cooper published an inventory of the risk factors that might have contributed to Fixx’s death. Granted access to his medical records and autopsy, and after interviewing his friends and family, Cooper concluded that Fixx was genetically predisposed (his father had a heart attack at 35 and died of another at 43, and Fixx himself had a congenitally enlarged heart), and had several lifestyle issues. Fixx was a heavy smoker prior to beginning running at age 36, he had a stressful occupation, he had undergone a second divorce, and his weight before he took up running had ballooned to 220 pounds (100 kg).  A carved granite monument — a book with an inscription to Jim Fixx from the people of Northeast Scotland — now stands in Hardwick Memorial Park in Hardwick, Vermont.