Competing for Great Britain in the 1968 Summer Games in Mexico City, Hemery won the 400 hurdles in a world record time of 48.11. Four years later, Hemery claimed the Bronze Medal in the 400 hurdles and the Silver Medal in the 4X400 relay at the 1972 Summer Games in Munich, Germany.

In between the two events, he won the 110M hurdles at the 1970 British Commonwealth Games, served as team captain for England and Britain at various international meets, and was named the European Runner of the Year and the BBC Sportsman of the Year in 1969.

Hemery previewed his future success on the international level during his four years at Boston University, where he captured the 1968 National Championship in the 400 hurdles, earned All-America recognition in 1966 and 1968, and received the Gordon "Mickey" Cochrane Award as the University's Outstanding Male Athlete during his sophomore year in 1966.

He won three IC4A Titles, capturing the indoor 60-yard hurdles and the outdoor 120-yard hurdles in 1966, and the outdoor 440-yard hurdles in 1968. In addition, he copped the 110-yard and 440-yard hurdles at the 1966 and 1968 Outdoor New England Championships.

Despite graduating in 1968, Hemery holds three existing University records; the 60-yd hurdles (7.1), the 600-yd run (1:09.8), and the 400-m hurdles (48.1).

Inducted into the University's Athletic hall of Fame in 1972, Hemery received his Master's Degree in Education from Harvard, and his Doctorate Degree in Education from Boston University. He later served as the University's men's and women's track coach, and earned New England Coach of the Year honors in 1979.