Ms Platt was in charge of the organisation for 10 years before departing in August to focus on her other role as non-executive director of the FCA.
The Chancellor has appointed Mr Ackerley as her replacement.
Mr Ackerley said: “Like many people across the UK, I grew up with NS&I: my first savings were in Premium Bonds, which were bought for me as a child.
“It is a brand I have long admired – particularly for how it has transformed itself into a 100% direct business without losing the trust and loyalty of its customers. I’m proud to be given the opportunity to lead NS&I and look forward to joining the team.”
He has been director of investments at Barclays Bank for over 5 years and was managing director for investments & pensions at
Virgin Money before that, according to LinkedIn.
He also worked for Axa and Sun Life International earlier in his career.
Simon Kirby, Economic Secretary to the Treasury, said: “Ian Ackerley’s extensive experience in retail financial services and effective leadership skills make him the ideal candidate to be the next Chief Executive of NS&I.
“NS&I sits at the heart of our savings sector and Ian will play a key role in shaping the next phase of NS&I’s digital transformation so that it continues to deliver for millions of savers across the UK."
The amount savers invested with NS&I grew from £73 billion in 2006 to £134 billion at the end of 2015 and saved the taxpayer over £4 billion, according to NS&I’s figures.
In 2006, half of sales came through the Post Office, whereas it has now become a 100% direct business, officials said.