The Scarborough-born Conservative was told by the Prime Minister that she had "lost confidence in his ability to serve in the role of Defence Secretary and as a member of her Cabinet".
He is to be replaced by International Development Secretary Penny Mordaunt, who will continue in her role as Minister for Women and Equalities.
A former Chief Whip who was educated at schools in North Yorkshire and went to university in Bradford, Mr Williamson had served as Defence Secretary since 2017.
An inquiry by Cabinet Secretary Sir Mark Sedwill was launched after information from secret discussions about Chinese telecoms firm Huawei's involvement in the development of the UK's 5G mobile network was printed in the Daily Telegraph.
Mr Williamson was listed in the Telegraph as being among a small group of ministers whose warnings about Huawei's involvement were overruled by the Prime Minister.
A Downing Street spokesperson said: “The Prime Minister has this evening asked Gavin Williamson to leave the Government, having lost confidence in his ability to serve in the role of Defence Secretary and as a member of her Cabinet.
“The Prime Minister’s decision has been informed by his conduct surrounding an investigation into the circumstances of the unauthorised disclosure of information from a meeting of the National Security Council.
“The Prime Minister thanks all members of the National Security Council for their full cooperation and candour during the investigation and considers the matter closed.”
In her letter to Mr Williamson, Prime Minister Theresa May said: "Thank you for your time this evening. We discussed the investigation into the unauthorised disclosure of information from the National Security Council meeting on 23 April.
"This is an extremely serious matter, and a deeply disappointing one. It is vital for the operation of good government and for the UK's national interest in some of the most sensitive and important areas that the members of the NSC - from our Armed Forces, our Security and Intelligence Agencies, and the most senior level of Government - are able to have frank and detailed discussions in full confidence that the advice and analysis provided is not discussed or divulged beyond that trusted environment.
"That is why I commissioned the Cabinet Secretary to establish an investigation into the unprecedented leak from the NSC meeting last week, and why I expected everyone connected to it - Ministers and officials alike - to comply with it fully. You undertook to do so.
"I am therefore concerned by the manner in which you have engaged with this investigation. It has been conducted fairly, with the full co-operation of other NSC attendees.
"They have all answered questions, engaged properly, provided as much information as possible to assist with the investigation, and encouraged their staff to do the same. Your conduct has not been of the same standard as others.
"In our meeting this evening, I put to you the latest information from the investigation, which provides compelling evidence suggesting your responsibility for the unauthorised disclosure. No other, credible version of events to explain this leak has been identified."