As a young associate at Simpson Thacher & Bartlett in New York City working on a massive antitrust case, Corey Lee knew it wouldn’t be easy to land more responsibility, visibility and unique experiences, so he actively looked for a gap to fill.

And before long: an opportunity, as the junior partners on the case asked if Lee wanted to tag along to the CEO of MasterCard Inc.’s office to collect documents. The credit card company, along with Visa, faced allegations from American Express Co. and Discover Financial Services that they’d blocked banks from issuing competitors’ cards.