In July 2000, he joined Apple as Product Line Manager for Mac OS X Worldwide Product Marketing. Zelenka has since worked on the initial public beta of Mac OS X, the release of Mac OS X Server, and various hardware and software initiatives including Xserve, Xsan, and Apple Remote Desktop. Zelenka currently manages Worldwide Product Marketing for Core OS software used by iOS, macOS, tvOS and watchOS. He manages Core OS technologies including UNIX, networking, IPv6, network file services, Mobile Device Management, Caching Server, and Apple File System.