King will join the car-listings business in the coming weeks, after he finishes his current role as chief executive of Trading Post, a specialist classifieds business in Australia, owned by advertising and directories company Sensis.
The group's flagship brands are the Trading Post weekly print publication and its companion website, www.tradingpost.com.au. Other titles include Autotrader, Things That Go, Local Trader and Collectormania.
King's previous roles include chief operating officer of Sensis Classifieds and general manager of online advertising at Sensis.
Carolyn McCall, chief executive of Guardian Media Group, said: "John has extensive experience of building and leading successful print and online classified advertising businesses. He also has a track record of delivering high levels of growth in very competitive industries and market segments."
King said: "I look forward to working with the team to build on the company's leading position, and making sure that we continue to deliver the highest levels of service for our customers."