CC Rogers & Wells is acting on its first major showcase trial since merging with Clifford Chance.
The firm is representing long-term client Mastercard as it begins its battle with the US Department of Justice (DOJ).
Along with Visa, Mastercard stands accused of violating anti-trust laws by limiting competition through tie-ups with the banking industry.
It is understood that Rogers & Wells received some $10m (£6.7m) in fees from Mastercard in its financial year ending 1999.
The DOJ, which recently successfully sued software giant Microsoft for anti-competitive practices, launched its investigation into the credit card companies in 1998. The trial got under way in New York last week.
Mastercard is a major client of the New York-based firm, which merged with Clifford Chance last year.
Rogers &Wells is also representing it in another US case, a class action lawsuit brought by retailers seeking damages for alleged anti-trust violations.
The DOJ case is being led by New York-based anti-trust partner James Benedict.
Fellow anti-trust partner Kevin Arquit is also working on the case, as are Washington-based anti-trust partners Kenneth Gallo and Steven Newborn.
Visa is being represented by San Francisco-based Heller Ehrman White & McAuliffe.
The firm has also recently been retained by Microsoft to act in private anti-trust litigation following the DOJ's findings.
Anti-trust and litigation partner M Laurence Popofsky is leading the case. Fellow partner Stephen Bomse, who also acts for Microsoft, is working with Popofsky.
Mastercard and Visa are arguing that only competitors such as American Express would benefit if the DOJ rules against them.
A Clifford Chance spokes-man confirms that the firm is acting for Mastercard but declines to comment further.