Even by the dizzying standards of New York City philanthropy, a recent $6.24 million donation to the Henry Street Settlement on the Lower East Side was a whopper — the largest single gift from an individual to the social service group in its 125-year history.

It was not donated by some billionaire benefactor, but by a frugal legal secretary from Brooklyn who toiled for the same law firm for 67 years until she retired at age 96 and died not long afterward in 2016.

Her name was Sylvia Bloom and even her closest friends and relatives had no idea she had amassed a fortune over the decades. She did this by shrewdly observing the investments made by the lawyers she served.

“She was a secretary in an era when they ran their boss’s lives, including their personal investments,” recalled her niece Jane Lockshin. “So when the boss would buy a stock, she would make the purchase for him, and then buy the same stock for herself, but in a smaller amount because she was on a secretary’s salary.”