Cahill spent his childhood primarily in Waukesha, Wisconsin. He attended the University of Wisconsin—Madison on a swimming scholarship. He was friendly in college with Steve Miller and Boz Scaggs.

Along with professional long-distance driver Garry Sowerby, Cahill set a world record for speed in driving the entire length of the American continents, from Ushuaia in Tierra del Fuego in southern Argentina up along the Pan-American Highway to Prudhoe Bay, Alaska in twenty-three days, twenty-two hours, and forty-three minutes. This trip was the source material for his book Road Fever.

He has written several books recounting his Adventure travel experiences and blends his own brand of humor into his stories. He is a frequent contributor to National Geographic Adventure magazine.

Cahill lost his wife, Linnea Larson, to a traffic accident in April 2008.