Hicks, formerly Nick’s marketing chief, was upped to president, content development and production at the Nickelodeon Group in 2012.
According to various US reports, he has resigned after a four-month medical break, and Nick has no immediate plans to replace him.
Hicks had control over live-action, animation development and production for all platforms and Nick-branded networks.
Last year, former Nick chief Cyma Zarghami was elevated to president of a new Viacom Kids and Family Group, which houses Nick with older-skewing networks such as CMT and TV Land.
Deadline, which first reported the story, said there had been friction between her and Hicks, though also noted she had spoken highly of him in an internal memo confirming his departure.
Under Hicks, Nick’s ratings have improved after a period of decline, taking the pay TV channel back to number one in the ratings charts.
“Russell Hicks has decided to step down from his position effective immediately,” a statement issued to US trades read.
Hicks has previously worked at Cartoon Network and Warner Bros.
Hicks is the latest channel chief to exit Viacom, with his departure coming a month after Michele Ganeless left Comedy Central. Various other top-level network execs have left since a major company restructure.
The future of Viacom’s top tier management rages on, meanwhile, with Philippe Dauman and other members of the Viacom board clashing with company controller Sumner Redstone and his daughter, Shari Redstone, over the sale of a stake in film studio Paramount Pictures.