Mr Johnson said his successor had “the right plan to tackle the cost-of-living crisis” and “unite our party”.
In her victory speech after a 57% to 43% victory over Rishi Sunak, Ms Truss praised the work done by “my friend” Mr Johnson.
Congratulations to @trussliz on her decisive win. I know she has the right plan to tackle the cost of living crisis, unite our party and continue the great work of uniting and levelling up our country. Now is the time for all Conservatives to get behind her 100 per cent.
— Boris Johnson (@BorisJohnson) September 5, 2022
She said: “Boris, you got Brexit done, you crushed Jeremy Corbyn, you rolled out the vaccine and you stood up to Vladimir Putin.
“You were admired from Kyiv to Carlisle.”
Mr Johnson quit as Tory leader after a revolt by senior ministers – including former chancellor Rishi Sunak – in protest at a series of scandals including groping allegations against former deputy chief whip Chris Pincher.
The Prime Minister said: “I have been proud to serve as leader of the Conservative Party for the last three years, winning the biggest majority for decades, getting Brexit done, overseeing the fastest vaccine rollout in Europe and giving vital support to Ukraine.”
He said Ms Truss had scored a “decisive win” – although her margin of victory was smaller than that enjoyed by predecessors dating back to Iain Duncan Smith.
“I know she has the right plan to tackle the cost-of-living crisis, unite our party and continue the great work of uniting and levelling up our country,” Mr Johnson said.
“Now is the time for all Conservatives to get behind her 100%.”