Police Constable Robert Brown, known as Bob, joined the Metropolitan Police force as an 18-year-old in 1968.
He finished his career in Croydon Borough, where he was born.
Equipped at first with just a wooden truncheon and whistle he spent his first 15 years working in Camden Borough.
Speaking after his colleagues formed a guard of honour for him for his final shift, he said: "I'm overwhelmed that everyone turned out today for this.
"I have always wanted to serve and help people and that's been my ethos through my career. I wanted to help people.
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"I've always been a Police Constable. Not everyone can be a commander. I enjoyed what I was doing.
"I will miss the people immensely but not the job. It's run its course for me," Brown was quoted as saying by the BBC.
Croydon Borough Commander Chief Superintendent Andy Tarrant said: "It is only fitting that Bob should complete his career in the area that he was born in.
"An operational officer virtually to the end of his career, Bob will be missed by all his friends and colleagues."