The move comes as part of a wider management restructure which will also see global head of distribution Julian Lyne promoted to chief commercial officer and maintain responsibility for client relationships.
Mr Custard joins next week from UBS Global Asset Management, where he was group managing director and head of investment solutions. He was previously head of multi-asset at Schroders.
Newton has operated without a CIO since 2015, when the long-serving Simon Pryke left after 19 years. At the time, Newton opted not to directly replace Mr Pryke, instead promoting Charles French to head of investments.
Mr French retains his title in the restructure, and will now report in to Mr Custard.
Newton chief executive Hanneke Smits said the BNY Mellon subsidiary would now seek to expand further on a global basis.
"It builds on our investment philosophy and process and reflects the board's commitment to reinforcing governance," she added.