Underhill, who will be based in the company’s London office, will be responsible for the client services and business operations for AppNexus across Europe, the Middle East, and Africa.
He was previously global commercial director and director of product and operations at London-based Phorm and, prior to that, worked as chief operating officer for Advertising.com/AOL.
Underhill joins the 40-strong London team, which has grown from two when it opened in October 2011.
Meanwhile, Paine joins the company’s Madrid-based team and will lead a new group for AppNexus focusing on major global accounts and partnerships.
Before joining AppNexus, he spent 10 years at DoubleClick/Google where he was most recently global agency business leader.
AppNexus president Michael Rubenstein said, "It’s a real coup to have Jerome and Brandon join AppNexus. Their unique capabilities speak volumes about our commitment to building the world’s best ad tech company with a strong presence in all vital global markets."
The appointments have been announced at this week’s first European AppNexus Summit, held in London.