Denis Glaccum was one of the first members of the USEA and rode in his first event in 1956. Four years later, Glaccum competed in the 1960 Olympic Selection Trials. In 1976, Glaccum started the event which would become the Chesterland Three-Day Event, and 10 years after that he formed Fair Hill Equestrian Events, Inc., which now goes by the name Plantation Field Equestrian Events and is responsible for organizing the events at Plantation Field. Glaccum also pioneered the use of portable cross-country jumps at Fair Hill, a practice that has become mainstream in modern eventing.

Glaccum co-founded the American Horse Trials Foundation, which allows riders to collect tax-free donations. He served for many years on the USEA Board of Governors in a variety of positions and was involved with the introduction of computers to the organization and the headquarters’ move to Morven Park. He is also a former member of the United States Equestrian Team’s Three-Day Event committee. In the early 2000s, Glaccum assisted in writing the curriculum for the Course Designers’ Clinics.

Glaccum holds the honor of having competed in events longer than any other American, competing from 1956 to 2012.