[1] He graduated with a Bachelor of Veterinary Science from the University of Sydney in 1954 and then worked briefly as a cattle veterinarian and had a short career in the animal industry.
[2][4] He often travelled to North America to learn about veterinary practices relating to small animals there and was a guest lecturer at institutions including the University of Guelph in Canada and the Washington State University College of Veterinary Medicine.
They have both been noted patrons of animal- and biodiversity-related causes (for which Mary received a Medal of the Order of Australia in the 2024 King's Birthday Honours), along with the Australian Brandenburg Orchestra.
[2][7][8][9] He died aged 82 on 24 June 2013 in the Blue Mountains village of Mount Wilson; he was privately cremated.
[3] Holt received the World Small Animal Veterinary Association's Award for Service to the Profession in 1998,[5] a Waltham Award for International Service from the American Academy of Veterinary Nutrition, and honorary memberships in several organisations such as the American Animal Hospital Association.