Jonathan Mainwaring, from Surrey, runs the 5k Parkrun every weekend but when he celebrated clocking up his 250th run he had no idea it would lead to an emotional family reunion.
A photo of the runner doing the Mobot pose, inspired by Mo Farah, was shared on Ottershaw Parkrun's Facebook page in December.
Unbeknown to Mr Mainwaring, his family in Manchester had traced some relatives to the area and searching for his surname and 'Ottershaw' led them to Parkrun, then to their missing relative.
Mr Mainwaring, 53, told Sky News the discovery came "completely out of the blue".
He said: "I was shocked and quite emotional. Seeing a few photos of relatives who looked exactly like my dad was an emotional moment too.
"It would have moved him too, he would have been delighted to meet them. He lived his life thinking he was alone, and making his own family.
"They are so lovely, we couldn't have picked a better family. They would have been pretty pleased with him too."
Mr Mainwaring's father Jimmy died around 10 years ago, thinking he was an orphan without any family.
As a young boy, Jimmy had been put into an orphanage in the North West, then moved to the Gordon Boys' School in Surrey, following the outbreak of the Second World War.
He eventually settled in Ottershaw, where repeated attempts to trace his family were unsuccessful.
The records had been destroyed, so he never found out why he was put into an orphanage.
After Mr Mainwaring's 'Mobot' was posted on the Facebook page, his wife Lisa was contacted by a long-lost family member - the daughter of Jimmy's sister.
The relative had been looking into Jimmy's background and through a genealogist in the US found he had children called Elaine and Jonathan.
Searching for 'Jonathan Mainwaring' and 'Ottershaw' led them to the Mobot photo, and they were able to find Mrs Mainwaring on Facebook to get in touch.
She said: "My hubby isn't on Facebook - so if parkrun hadn't shared the photo or he'd done his 250th Parkrun at another location, they would never have found him. It was such a coincidence."