For years, Netflix Inc. NFLX 2.40%▲ boss Reed Hastings was emphatic that the streaming giant wouldn’t show ads in its programming. Late last year, the company did a 180. Netflix didn’t just want to enter the ad business—it wanted to do so in a hurry.

The company assembled engineers to sound them out on the technical challenges. Executives quickly decided they would initially need an external partner to supply technology and sales power.