Peat studied at Montrose Academy in Scotland, and was then apprenticed to a local solicitor but he did not enter the legal profession. Instead, he moved to London in 1870 and was hired as a junior accounting clerk for a London firm, becoming a partner in 1876.
Peat born in Forebank, St Cyrus, Kincardine, Scotland. He was the second son of James Peat and Margaret Barclay (of the banking family that built Barclays, one of England's largest banks).