Which cigarette brand was Sterling Cooper's biggest client?

Sterling Cooper’s biggest client was Lucky Strike. The cigarette company’s account is so financially important that the agency’s fortunes repeatedly depend on keeping its business.

Lucky Strike is central from the pilot episode, “Smoke Gets in Your Eyes,” when Don Draper must create a campaign for the brand amid growing public concern about the health effects of smoking. The episode dramatizes the real advertising practice of turning a product characteristic into a memorable selling point, leading to the famous “It’s Toasted” campaign in the show.

The account also drives later plot developments. When Sterling Cooper loses Lucky Strike, the agency faces severe financial pressure and must downsize. The brand’s importance is emphasized through Roger Sterling’s long relationship with the client and the company’s willingness to tolerate difficult behavior from Lucky Strike executive Lee Garner Jr. Lucky Strike is therefore more than a recurring brand—it is the agency’s commercial backbone.