Previously, Dr Brash has been a member of Parliament, having resigned as governor of the Reserve Bank in 2002 to enter Parliament, with the stated motivation of reducing the gap in living standards between New Zealand and Australia.
In 2003, he was elected leader of the National Party (then in opposition) and used his position to move public debate forward on many key issues.
"Our advisory board members are selected for their integrity and compatibility with our values. They use their best judgment and expertise to offer sound advice and their collective opinion to the directors and management team of Stewart Group and associated company Boutique Advisers Alliance.
"Don's expertise and knowledge of the financial industry will be invaluable to the direction of the Stewart Group and we are delighted to have him join the board."
Dr Brash is the chairman of the New Zealand subsidiary of the Industrial and Commercial Bank of China (ICBC), now the largest bank in the world.
He has been a director of many companies, including Westpac Merchant Finance and Cavalier Corporation in the 1970s and 80s, and the ANZ Bank (in New Zealand), Transpower and Oceania Dairy more recently.
He has served as an adjunct professor of banking in the Business School at AUT University in Auckland and as an adjunct professor in the Faculty of Law and Management at La Trobe University in Melbourne.
He has a PhD in Economics from the Australian National University.
Dr Brash said he was delighted to join the family-owned Stewart Group, which continues to raise the bar in the provision of financial advice to New Zealanders.
"I've had a lot to do with the financial sector for most of my career, but few companies have so impressed me with their integrity and professionalism."
Fellow board member Mr Mancell, managing director of FYG Planners and chairman of the Global Association of Independent Advisers, said: "It's great to work with a company that strives for best practice and bringing on board Dr Brash further diversifies the advisory board.
"Stewart Group and the Boutique Advisers Alliance is the only advisory firm I am aware of with a former Reserve Bank governor on its bench."
Stewart Group is the asset consultant for the Asset Class KiwiSaver funds administered by Booster. It was the founding member of the Boutique Advisers Alliance, a service provider to independent financial advisers, and a founding member of Global Association of Independent Advisors (GAIA) – a Geneva-based association for financial advisers.