Halstead was given a Kodak Duaflex camera by his parents for Christmas when he was 15 years old, and began photojournalism while in high school. Two years later, he became Life magazine's youngest combat photographer covering the Guatemalan civil war. He studied at Haverford College for one year, before dropping out to work in Dallas. He was subsequently drafted into the US Army and served for two years. Upon his return from military service, Halstead joined UPI and ultimately worked there for more than 15 years. During the Vietnam War, he was UPI's picture bureau chief in Saigon.
Time designated Halstead as their Senior White House Photographer in 1972. He later accepted a contract with them that same year which lasted for the next 29 years. Halstead was one of the six photographers who accompanied Richard Nixon on his historic trip to China in 1972. His photographs have appeared on 49 Time covers, more than any other photographer. During this period he also worked as a "Special Photographer" on films to produce photographs used in advertising materials for the major commercial studios. The films he worked on included Goodfellas, Memphis Belle, Shaft, Black Rain, Dragon, Dune, Conan the Barbarian series, Greystoke, and Cliffhanger.