Sir James Lowther was one of the most important financiers of his day. According to the figures given by P. G. M. Dickson, Lowther's holdings of stock in the 1740s were the largest of all among the 7 per cent of proprietors who lived outside London and the home counties (although, strictly speaking, he should perhaps not qualify for this category since he was born in the capital and spent much of his life there). What is probably an apocryphal story has it that the Earl of Sunderland, First Lord of the Treasury 1718–21, intended to offer Lowther a Treasury post because of his reputation in financial affairs. He apparently changed his mind on finding Lowther to be ‘a mean fellow’.

Not a great deal is known of Lowther's activities as a financier beyond his role in the South Sea Company during the 1730s. He was amongst those who believed that the Company should not actually engage in trade, and a proponent of a scheme debated in the early 1730s to have part of the stock turned into annuities. The extent to which Lowther was the instigator, or simply followed where others led, is hard to determine with accuracy. He is known, however, to have been involved with the scheme both in Parliament and at meetings of the Company's general court.

The conversion scheme originated at a meeting ‘at the Ship Tavern by Temple Bar’ on 22 March 1732. Lowther was amongst those who ‘agreed to oppose the directors’ scheme for paying a million of Bonds and annihilating 6¼ per cent of the stock'.