Steward has been in his current role at RKCR/Y&R, which works with brands including Marks & Spencer and Lloyds Bank, since late 2013. He is one of three managing directors, alongside Vicky Jacobs and Rupert Williams.
Steward joined the agency in 2002 as a junior account manager, and became the head of new business of RKCR/Y&R and Saint in 2007. Saint, RKCR/Y&R’s digital shop, was merged into the main agency in April 2012. He was promoted to client services director in May 2012.
Steward was an account manager at JWT London between 2000 and 2001.
A spokeswoman for DigitasLBi declined to comment.
Ben Kay, the chief executive at RKCR/Y&R, said: "We're lucky to have had Matt at the agency for more than 12 years and wish him all the best on his next adventure."