The Prime Minister has also sacked his Business Secretary Andrea Leadsom, Environment Secretary Theresa Villiers, and Housing Minister Esther McVey, who attended cabinet - but Number 10 has said the cabinet will have the same gender split at the end of the reshuffle, with gender balance in junior ministerial posts to create a pipeline of experienced women for the future.
And the sacking were rounded off with the removal of Attorney General Geoffrey Cox, who was removed minutes before being due to take questions in the Commons. Culture Secretary Nicky Morgan is also leaving the cabinet as planned, having been given a peerage so she could carry on the role after standing down as an MP.
Reports suggest that Mr Johnson felt blindsided by a commitment in the power-sharing compromise struck by Mr Smith that meant British army veterans of the Troubles could be pursued for alleged abuses.
The Skipton and Ripon MP, who was appointed to the role in July 2019 after serving as Theresa May’s chief whip, tweeted: “Serving the people of Northern Ireland has been the biggest privilege. I am extremely grateful to Boris Johnson for giving me the chance to serve this amazing part of our country.
“The warmth and support from people across Northern Ireland has been incredible. Thank you so much.”
The sacking raises questions over whether the UK Government will follow through on the deal struck last month. There were warm tributes to Mr Smith from across the UK and Northern Irish parties, and an angry reaction from Colum Eastwood, the leader of the SDLP.
"Thank you Julian Smith for your tireless commitment to devolution, for the work you've done for victims of historical institutional abuse and for securing much needed resource for Derry," the Foyle MP tweeted. "Sacking the most successful SoS in a decade shows Johnson's dangerous indifference to us."
Ms Leadsom, a former Tory leadership contender tweeted: "It has been a real privilege to serve in Government for the last six years, and in BEIS [the Department for Business, Energy and Industrial Strategy] for six months. I now look forward to focusing on my constituents and on my 20+ year campaign to see every baby get the best start in life.
"I’m particularly proud of my work in BEIS on net zero and the #CountDowntoCOP and on making the UK the best place in the world to work and to grow a business."
Ms McVey's removal will make her successor the 10th Housing Minister in a decade. She posted on twitter: "I’m very sorry to be relieved of my duties as Housing Minister. I wish my successor the very best and every success.
"I’m very grateful to the Prime Minister for having given me the opportunity to serve in his government and he will continue to have my support from the back benches."
Transport Minister George Freeman and Universities and Science Minister Chris Skidmore have also left the government, with Mr Skidmore tweeting that he had "got a promotion in the reshuffle to be a better dad".