The Labour leader was driven to the meeting, the subject of intense media interest after a draft of the manifesto was leaked, by officers from the Metropolitan Police's royalty and specialist protection unit when his vehicle was mobbed by journalists.
By moving to the left, Labour has cleared the way for Prime Minister Theresa May to take control of the centre ground of British politics and appeal to traditional Labour supporters who backed leaving the European Union, a strategy that has handed her ruling Conservatives a runaway lead in the opinion polls.
"We have just unanimously agreed the contents of it. We've amended a draft document in the most informed, interesting and sensible debate".
Full details and costings for all the policies would come in the next few days, Corbyn said.
It was an upbeat end to a tricky day for the party.
In the draft manifesto, Labour said it would reform corporate governance, rein in "boardroom excess" by setting a 20:1 limit on the ratio between the lowest and highest paid in companies with government contracts and protect small businesses.
The left-wing commentator, founder of the commentary website Novara Media, expressed his support for the key policies outlined in Labour's manifesto.
A 43-page document was circulating on Wednesday night, but senior Labour sources said the draft discussed at the meeting on Thursday was longer.
At the poster launch near Waterloo in south London, Lavery and his co-campaign coordinator, Andrew Gwynne, unveiled a truck-mounted poster showing the slogan "The Tories have held back Britain long enough".
"Because you don't lose the courage to fight for what you believe in, you don't stand back and say this is now not for me, it's too hard, that you continue to fight and support and champion what really matters to the future of this country".