Until his retirement Dr Jefferson was the owner of Maine Equine Associates, an ambulatory equine practice. He is originally from Pelham, New York, miles from the nearest horse. His interest in horses began at Cornell University where he earned his veterinary degree.

Before college he served as an enlisted US Marine for three years. Dr J is a member of the Vineyard Christian Fellowship of Mechanic Falls and is part of a team that visits jail inmates. He has been a Literacy Volunteer, working with adults who have difficulty reading. He served as President of the Lewiston-Auburn Rotary Club and the Maine Veterinary Medical Association. He is an active board member of the Maine State Society for the Protection of Animals. For the last few years he has been teaching large animal medicine to vet tech students at York County Community College in York, Maine.

David has been married to Bonnie for 56 years. They have two grown children. Their son, Jim, lives in mid-coast Maine. He is an ecological cruise boat captain in Alaska during their tourist season. Their daughter, Tally, lives with her husband, Ernie, in St. Petersburg, Florida, where they are in the real estate business together

David enjoys writing. His first book, Maine Horse Doctor, was published in 2017 with solid reviews on Amazon.