The 68-year-old Liverpool-born ophthalmologist, who has served a total of 45 years in health service, has retired from Sheffield Teaching Hospitals as consultant ophthalmic surgeon after spending almost four decades in the city.
And there was one word to describe his feeling as he put his phone down on his last day at work - "strange".
"It's the only word I can use. I particularly remember I was doing a virtual clinic, phoning my patients. As I got to phone my last patient, I realised that was the last patient I was going to consult.
"A very strange feeling I must tell you. I told her she was my last patient, we sort of joked about it and after I put the phone down and I went, well that's it after 36 years," he said.
The University of Sheffield Medical School graduate was a prominent figure in his field. In 1995, he was awarded a personal Chair by the University after developing an ocular oncology service and research base.
Thanks to him, Sheffield was designated one of the three national supra-regional centres for treating eye cancer in 1997.
He also supported the Trust as a non-executive and clinical director, leading the expansion of the department from five to 25 consultants and recruiting and developing allied professionals.
In addition to that, he established one of the largest imaging departments in the country and supported the bid for University of Sheffield to offer one of the two national Orthoptic Degree courses.
Looking back, Professor Rennie said the camaraderie he had with his colleagues and his relationships with his patients are what kept him going.
"There are two things I will miss the most. First, my patients...they came to me for many, many years often from far and wide and a lot of them are like family. I'll miss them dreadfully.
"The other thing I miss the most is my colleagues. One of two have worked with me throughout my consultant career and we worked very closely together.
"I will miss the camaraderie, the banter and the pleasure of working with them."
And this year, Professor Rennie will be awarded the highest honour, the Honorary Fellowship, in recognition of his lifetime contribution to the Ophthalmology field, seemingly a fitting end to his illustrious career.
Previously, he was given a lifetime award in 2019 at the first UK Ocular Oncology Congress in Sheffield.
He said: "This has to be my proudest moment in my career because being a fellow is conferred to very, very few people indeed. This would have to count to be the most memorable moment in my career."
Asked about what his post-retirement plans would be, the grandfather-of-two said he would spend time on his hobbies such as gardening, fly fishing and astronomy.
"I love walking too and I have a cottage in Scotland which I try to go to whenever I can so I can spend time fishing and walking there."