After a short career as a Wall Street stockbroker, Capstick headed to Latin America, where he traveled widely while hunting, fishing, and mastering the Spanish language. A few years later he returned to New York, where he founded a business arranging professionally guided hunting trips. Shortly thereafter he took a position as hunting and fishing director of Winchester Adventures of New York, a subsidiary of the famous gun manufacturer. In that capacity he made his first trip to Africa in 1968. Subsequently he worked as a professional hunter and game ranger in Zambia, Botswana and Rhodesia.

Capstick started writing about his adventures in the late 1960s, and published numerous articles in various sporting magazines. In 1977 he published his first book, Death in the Long Grass, which became a major commercial success and established his reputation as a teller of hair-raising true adventure stories. He went on to become the world's bestselling hunting author with a string of books detailing his own adventures as well as those of other well-known hunters.

Capstick is frequently compared to Ernest Hemingway and Robert Ruark in discussions of influential African hunting authors. But some have questioned Capstick's use of "literary license" in his stories.

In early 1996 Capstick was a keynote speaker at the annual Safari Club International convention in Reno, Nevada when he collapsed in his hotel room and was diagnosed with exhaustion. He was immediately flown back to his adopted country of South Africa and underwent heart bypass surgery in a Pretoria hospital. He died just before midnight on March 13 of complications from surgery.

After a small private ceremony, his ashes were scattered over the Chobe River in northeastern Botswana.

The .470 Capstick rifle cartridge, developed by A-Square's Colonel Arthur B. Alphin in 1990, bears his name. His legacy is also saluted by The Dallas Safari Club's annual Peter Hathaway Capstick Hunting Heritage Award for the promotion of responsible hunting and wildlife conservation.