For the first time in two years, Nabs will also have a vice-chairman.
He is Mike Cornwell, the chief executive of TBWA\GGT Direct.
Nabs hopes the pairing will allow the workload to be spread more evenly - Duncan also sits on the IPA Council - while improving its links with below-the-line agencies.
John Bartle, the Nabs president, said: "Grant was top of our list of possible candidates. Everything I know about him is positive."
He added: "It's important that the chairman comes from the newer generation of managers who know their way around the business. Grant is universally popular and I know he'll do the job in a sympathetic and responsible way."
Duncan said: "The industry has been good to me and I'd like to give something back. My task is to put some weight behind Nabs and to harness help from people in the business in a coherent way."
The first big challenge to face Duncan, 45, who begins his two-year term of office in January, will be to ensure the success of the Nabs Big Bash, the fundraiser that will replace the annual boxing night.