Diversified mining and metals group Billiton has no formal panel, but has relationships with a number of firms.
The coup follows partner Paul Lush’s departure from the firm to become Billiton’s legal counsel in April.
Camerons acted for the group on the $327m (£219m) acquisition of shares in Sweet River Investments which secures a strategic stake for Billiton in the Brazilian iron ore industry.
Corporate partner Peter Smith led the team assisted by project assistant Malcolm Bennett and trainee Katie Broadbent.
Bennett was also an in-house counsel for a South African company in the Billiton Group before joining Camerons in November 1998.
He says: “We’ve done work for Billiton in the past but it has been smaller pieces of advice, not a major corporate transaction.
Asked why Billiton chose his firm for the deal, Bennett says: “Relationship factors undoubtedly played a role, but I would like to think that our ability to do the job was the crucial factor.
“I would certainly hope that Billiton would consider instructing us on similar scale transactions in the future.”
Bennett says that the work was offered to Camerons and it did not have to pitch for it.
Brazilian practice Mach-ado Meyer Sendacz & Opice and Cayman Islands firm Hunter & Hunter worked on local issues raised by the deal.