The position replaces and broadens the role of digital publishing director, previously held by Jim Murray-Jones, who moved to Haymarket last autumn.
King, previously director of research at Aegis-owned Diffiniti, will work alongside Craig Stevens, Trader Media's director of new media, to oversee the company's digital publishing and advertising, as well as business intelligence.
The role covers the group's entire digital portfolio, which includes extensions of titles such as Top Marques and Bike Trader as well as the flagship Auto Trader, whose website, www.autotrader.co.uk, attracts 2.3 million unique users a month, according to the publisher's figures.
The company, a division of Guardian Media Group, publishes 70 weekly magazines with a combined circulation of 1.5 million.
King headed research at Diffiniti for three years and was previously marketing manager at mobile content supplier at Superscape, which he helped to build from a start-up.