King takes over from Nat Solomon who became acting chairman when David Evans, former executive chairman, retired.
Soloman will continue at Grass Roots as director in a non-executive capacity.
King said: "After a year of significant change within the Grass Roots Group, the company is now well placed to take advantage of its scale and market-leading position.
"I very much look forward to overseeing the next stage of its development. We have a strong executive team in place and I am eager to be a part of the exciting plans for future growth."
Prior to joining Grass Roots, King was managing partner at Ernst & Young, a role he took up in 2006 after being at the company for more than 30 years.
The agency also made a number of changes to its senior management last year. Grass Roots chief executive Richard Bandell rejigged the group's senior management team, following Alan Newton's redundancy