Now, exactly 45 years since his first day, he is hanging up his stethoscope to spend more time "smelling the roses".
The 67-year-old will retire from the Mornington Veterinary Clinic at the end of the week after marking his professional anniversary on Sunday.
He has been working out of its sister site, the St Kilda Veterinary Clinic, for about a month while the Mornington clinic is temporarily closed.
Dr Keenan said the only reason he could last so long in the profession was because he took great satisfaction from his work.
People had deep bonds with their pets and he felt honoured to be included in that when medical care was needed.
He was always honest about the reality of the situation, including treatment options and costs, and clients were grateful for that.
Some clients kept "coming in year after year" with their pets, which he enjoyed.
He was most comfortable around cats and dogs, which made up almost every single animal they cared for.
When trying to inspect the wings, the bird touched both sides of the room, he said.
He graduated from Massey University in 1977 and spent the first year working in Levin, but had been in Dunedin ever since.
Although it was a satisfying job, there were times when it was tough.
The worst part of it was dealing with after-hours calls, which typically ranged from dogs eating chocolate to long queues of concerned phone inquiries.
Combined with the ongoing vet shortage making everyone in the industry busier, he decided it was"time to pull back a bit".
"What I really want to do is take some time to smell the roses."
He would still be doing irregular work to help keep pressure off the system and to keep him busy in his retirement.
