Age Active said it was informed of the gesture through a letter, with a copy of a will, stating it was one of the beneficiaries of a large bequest.
Thinking it was a hoax, the charity decided to investigate by calling a number for the listed law firm, which confirmed the financial details were correct.
David Schofield, who is a yoga and tai chi instructor for Age Active, said: “Initially, we were convinced that it was a hoax and it was just somebody wanting the bank account details.”
The donor was later revealed to be James Fredrick Cousins, who had an engineering business in the city but didn't leave behind any family. He had left more than £1 million to be shared beween 11 small South Yorkshire charities.
Age Active’s founder, Suzanne Duraid, decided to visit his property in Grenoside, in the hope she would find somebody to whom she could express her gratitude.
There, she spoke to his immediate neighbours, who explained that the benefactor would have spent a considerable amount of time selecting the given charities, as he was thought to be a 'very meticulous man'.
She said: "The gift will ensure that Age Active will remain financially stable for the foreseeable future.
"We were very grateful because he saved us. It couldn't have come at a better time!"
Charity helps over-50s to get active
Founded in 1992, Age Active has helped more than 200,000 over-50s to get active.
According to recent data from Age UK, the number of over-50s experiencing loneliness is set to reach two million by 2025/6. This compares to around 1.4 million in 2016/7 – a 49 per cent increase in 10 years.
With over 500 members aged 50 to 96, there is a varied program of classes to suit different age groups.
What began as a single keep fit class with about 30 keen pensioners has blossomed into a flourishing organisation with 19 classes a week.
The classes take place at Victoria Hall in Sheffield city centre, located on Chapel Walk.
Age Active offers pay-as-you-go membership at just £5 per class, with no commitments. To find out more, call 0777 275 8146 or email [email protected]