Lacy was born in Edgbaston, Birmingham, the son of John Pierce Lacy of Wexford in Ireland, and grandson of James de Lacy of Garryrichard. These Lacys descend from the O'Laitheasa family of County Wexford, a name which is rendered Lacy in English.
Lacy was educated at St George's College, Weybridge. He practised as a stockbroker in Birmingham, becoming a partner in Cutler & Lacy and chairman of the Birmingham Stock Exchange. He founded and chaired the British Trusts Association in 1917 and the British Shareholders Trust in 1921.
In 1898, Lacy married Ethel Maud, daughter of James Finucane Draper, of St Helier, Jersey. They had one son, Maurice John Pierce Lacy (2 April 1900–22 April 1965), who succeeded as 2nd Baronet, and five daughters.
- Obituary, The Times, 27 December 1956
- De Lacy-Bellingari, Edward de Lacy-Harnett, The roll of the house of Lacy, 1928, p.87