Boris Johnson stopped to speak and take photos with a few members of the public as he said Conservative mayor Mr Street would "boost skills and drive long term growth in jobs".
The pair knocked on a few doors and handed leaflets out at a Kwik-Fit garage following a bike ride along a canal in Stourbridge on Wednesday.
Former John Lewis boss Mr Street is hoping to win a second term as mayor but will need to beat competition from Birmingham Labour MP Liam Byrne, with Thursday's poll expected to be a battle between the two major parties.
Labour leader Sir Keir Starmer, meanwhile, was in Birmingham where he talked up the chances of Labour’s mayoral candidate Liam Byrne. The Labour leader said Birmingham MP Mr Byrne had a “vision of a new future for the heart of Britain”.
He said these elections were about a “rebuild” for Labour ahead of the next general election, but warned it would take “longer than a year” for the party to get back on its feet.
WATCH: PM cycles in Black Country
It was Mr Johnson's second visit to the region in two weeks as parties have upped their campaign activity in the run-up to Thursday's poll.
Asked on his visit to Stourbridge what people had been saying about Mr Street, the PM said: "Well I found a lot of support obviously for Andy who I think has done an amazing job over the last few years.
"But it's going to be very close and I think it's a good opportunity for people to get out and vote tomorrow for Andy Street who has a fantastic agenda to take the West Midlands forward.
"I've been working with him for years now - he's brilliant at getting funding for projects here in the West Midlands but he also has a vision for the whole area.
"The connection between transport and extending the metro to Dudley, getting £1 billion of investment in Dudley town centre, getting Goldman Sachs, a huge American bank, to put their tech hub in Birmingham now.
"That's all part of what he wants to do to put in transport infrastructure investment, boost skills, and drive long term growth in jobs."
Questioned on whether a vote for Mr Street was the only way to get a fair deal for the people of the West Midlands, Mr Johnson said: "Of course - I think Andy Street has done an absolutely outstanding job and I would say, as I'm a backer of Andy's, that I think he would do a much better job than the other candidates.
"What he offers is a programme of growth and regeneration and knowing how to build new homes and create new opportunities on brownfield sites.
"If you look for instance at the HS2 interchange at Solihull, it's fantastic.
"That's an opportunity to create literally thousands of new homes, but not on greenfield sites, and it's a way of driving jobs and growth here for the long term - that's what Andy does."