In 1756 Macleane emigrated to Pennsylvania and practised medicine in Philadelphia in partnership with John Stuart, another Edinburgh graduate.
Before he took it up, however, his circumstances changed in two ways. The speculative boom in East India stock, beginning in April 1766, gave him an opening which he seized at first with great success. He organized large-scale combined operations on the London, Amsterdam, and Paris markets in which a number of prominent persons were concerned, including Lord Verney and William Burke;
Meeting this disaster ‘with the manliness of an Indian’, he won credit for accepting full responsibility for all his losses, and seems to have persuaded his creditors that he could serve them best if he were given the chance to retrieve his fortunes.
Obtaining in 1771 the sinecure of superintendent of lazarettoes, he applied for the Chiltern Hundreds on 8 May 1771, giving up his seat (he hoped only temporarily) to John Stewart.
Arriving in India in the autumn of 1773, he became intimate with Warren Hastings (who sent him on a confidential mission to Oudh) and impressed those with whom he came into contact, though he did not disguise his determination to make a rapid fortune and return to England.
He died with large debts still unpaid and with no assets except nebulous claims on the Nawab of Arcot (which were not honoured), and his reputation was further damaged by such particulars of his recent negotiations as became known to his associates.
