Eileen Thomas was Executive Director of the USEA and served the organization for 17 years from 1974 to 1991. Before living in the U.S., Thomas had spent 15 years working as secretary to the British Pony Club and was chief scorer at the Badminton Horse Trials. In 1974, Neil Ayer persuaded Thomas to move to the U.S. to help set up the office system for the USEA.

Thomas established the central horse registration and event registration programs, campaigned to have Training Points abolished, set the minimum age of 4 years old for competition horses, and revamped the grading system. Under her direction, the Omnibus grew from a tiny booklet to four major volumes a year and membership and starter numbers flourished immensely.

Horse welfare was always a top priority for Thomas and she campaigned for rule changes and for greater authority to be given to officials to ensure that abuse did not occur. In 1982, Thomas was awarded the prestigious Wofford Cup in recognition of her years of dedication to the sport.