He has been tasked with developing the agency’s full-service proposition – which includes creative and media planning and buying – helping to provide a better service to clients and delivering "sustained" growth.
Elliott replaces Luke Mugliston, who is stepping down after five years, and reports to Peter Reid, chief executive of parent company MSQ Partners.
Elliott is the latest high-profile appointment by The Gate, which hired Leo Burnett’s Beri Cheetham as executive creative director in November.
Reid told Campaign: "With Jamie and Beri, we hope for The Gate to be up there with the most respected and admired agencies."
The Gate’s clients include Chiltern Railways, Capital Group, the Royal Bank of Scotland, Close Brothers and Douglas & Gordon.
Elliott joined MullenLowe in 2005, when it was known as DLKW. He left in August last year just months after group chief executive Richard Warren.
Reid said: "[Elliott] is a modern, clever entrepreneurial leader. There are very few agency leaders who come close to his record in leading agencies through transformational change and in delivering sustained new-business success."
Elliott said: "I was looking for something quite different: independent, entrepreneurial in spirit and with the ability to move quickly. The Gate has creative and media already working together and sits in the dynamic, independent MSQ group. It meets the brief bang on and I’m relishing partnering with Beri to write a successful next chapter."