Ms McVey, who was eliminated from the leadership contest after finishing last in the first ballot on Thursday, said the former foreign secretary would be a "dynamic leader".
She said she had decided to back him because he had promised to deliver Brexit by October 31 and because he had agreed to back her agenda for "blue collar Conservatism".
The support will be seen as an extra boost to Mr Johnson, who goes into the second week of the campaign the clear favourite to succeed Theresa May.
However, none of the five other remaining candidates has shown any sign they are ready to withdraw to give him a clear run.
Writing in The Sunday Telegraph, Ms McVey said: "He has promised to deliver Brexit on October 31, deal or no deal, and has shown time and time again that he is a dynamic leader, capable of building a strong team around him that will deliver on his promises.
"Our country is crying out for strong, optimistic leadership and Boris is the man best equipped to take us out of the EU, to transform our country into an outward-looking, confident, self-governing nation, and to implement a policy agenda that will bring back the voters we have lost and ensure we don't allow Jeremy Corbyn's manifesto of economic carnage to plunge us back into the dark ages."
Mr Johnson is the only candidate in the remaining six who will not appear in the first television debate being held by Channel 4 on Sunday evening.
He has indicated he will participate in a BBC debate on Tuesday when the number of contenders will have been further whittled down, but said he was not keen on the format of the Channel 4 debate.
The broadcaster said it was "disappointed" and would represent his absence with an empty chair.
The remaining candidates are Foreign Secretary Jeremy Hunt, who was a distant second in the first round of the ballot, receiving 71 less votes than Mr Johnson.
He has insisted he had still not given up hope of winning in the final postal ballot of party members.
"I am the insurgent in this race," he told The Mail on Sunday. "I am in it to win it because we have to give the country better choices given the crisis that we're in now."
Environment Secretary Michael Gove came in third place but has described himself as the "Chumbawamba candidate", a reference to the band's hit Tubthumping, with the lyric "I get knocked down but I get up again".
He also sought to extend an olive branch to Mr Johnson after scuppering his last leadership bid in 2016, saying he would be happy to serve under him if he succeeded this time.
"I would absolutely work with Boris in any way that he wanted to work with me. No question. It is a different time requiring a different approach," he told The Sunday Times.