Stanley was the eldest son of twelve children born in Poland. He had emigrated to Britain with his father in 1913, the rest of the family following when the two had settled in Aldgate. He later said he was born in Oświęcim. He was granted a Polish passport in 1927. He gave a fairly ambiguous account of his early career but seems to have been employed from the age of 14 in garment sales and trading, especially in government contracts. He took his mother's name Koszyski.