Greeted by chants of "Boris! Boris!" from party supporters two days before an election which the Conservatives are predicted to lose heavily, he introduced the prime minister at a campaign event in London.
In a speech listing many of his own achievements, Johnson gave little personal endorsement to Sunak but focused on what he said were the dangers of the opposition Labour Party winning power.
"None of us can sit back as a Labour government prepares to use a sledgehammer majority to destroy so much of what we have achieved," he said.
Acknowledging that some might be surprised to see him, Johnson said he was glad to be asked to help by Sunak.
"Of course I couldn't say no," he added. "Whatever our differences they are utterly trivial by comparison with the disaster we may face if these so-called opinion polls are right."
Sunak took aim at Labour administrations across the UK in a campaign speech.
"Now, Boris is right to say now is the time for all conservatives to come together to deny Labour that super majority that Keir Starmer craves. We have 48 hours to save Britain from the danger of a Labour government," he said.
British Prime Minister Rishi Sunak urged voters to deny Labour a super majority. (EPA PHOTO)
"Now, I don't need to tell all of you in London what that looks like. Sadiq Khan whacking up council tax by 70 per cent, pricing motorists off the road with Ulez, allowing knife crime to soar.
"But it doesn't stop there. Labour have bankrupted Birmingham, the largest local authority in Europe and people there are paying the price with 20 per cent higher council tax."
Sunak added: "Now, it suits lots of people to say that the result of this election is a foregone conclusion but I know that it is not. Just 130,000 people switching their vote, giving us their support, is what it will take to deny Labour that super majority they want. Every single vote matters."
Meanwhile Keir Starmer said a big majority for Labour would be "better for the country", as it would give him a mandate to "seriously change" it.
However, Starmer also sought to play down expectations for how quickly some things might change if Labour wins power, as all the polls predict.
Tuesday night's Conservative Party rally was mostly notable for Johnson, one of British politics' most recognisable figures and a proven election winner, has spent almost the entire campaign on the sidelines, having quit frontline politics in 2023.
Labour's Keir Starmer has downplayed expectations ahead of polling day. (AP PHOTO)
Johnson won a big majority at the last UK election in 2019 before being forced to resign in 2022 by a Conservative mutiny which Sunak helped to start, and which exposed deep splits in the governing party, not least between Sunak and Johnson.
He has endorsed individual candidates in video messages but has not previously appeared at big campaign events.
Sunak appeared afterwards but not alongside Johnson on the stage, thanked his predecessor.
"Isn't it great to have our Conservative family united, my friends?" he said.
Sunak also urged Conservatives not to vote for Nigel Farage's Reform UK, adding this would "usher in" a Labour government.
with Reuters
