The former Top Gear host is staunchly anti-Brexit
The Prime Minister appeared with his neighbour and friend at the former Top Gear presenter's TV production company, where Clarkson spoke of his backing for staying in the EU.
It will be hoped Mr Cameron's appearance with Clarkson - for an informal chat over coffee, where they posed with cameras - will provide a shot in the arm for the Remain campaign.
The controversial television presenter, who was with his colleague James May, is likely to be able to reach parts of the electorate Mr Cameron and George Osborne cannot.
Speaking to Mr Cameron, Clarkson said: "It's an extraordinary thing that James and I only agree on three things, which is sandwich spread is delicious, the old Subaru Legacy Outback is a good car and Britain staying in."
He added: "I have not, with the greatest of respect, heard one politician say anything that's caused me to change my mind. There's huge numbers that don't understand and get confused. Really, it's my gut."
The meeting comes after a raft of bad poll results for the Remain campaign, which put Leave comfortably in the lead.
Clarkson came out in favour of remaining in the EU in a newspaper column in March, after careful consideration of the case on both sides.
He wrote in the Sunday Times: "Britain, on its own, has little influence on the world stage. I think we are all agreed on that. But Europe, if it were well run and had cohesive, well thought-out policies, would be a tremendous force for good."
He argued that the EU was in need of reform but that the UK was better off working from inside to change it saying: "But, actually, isn't it better to stay in and try to make the damn thing work properly? To create a United States of Europe that functions as well as the United States of America? With one army and one currency and one unifying set of values?"
Clarkson and Mr Cameron are friends, who live close to each other in Oxfordshire. At the height of the row over Clarkson's departure from Top Gear, after he punched a producer in a row over his dinner, Mr Cameron spoke to defend him.
Mr Cameron said: "He's a constituent of mine, he's a friend of mine, he's a huge talent. I see that he said he regrets some of what happened. Because he is a talent and he does amuse and entertain so many people, including my children who'll be heartbroken if Top Gear is taken off air, I hope this can be sorted out because it is a great programme and he is a great talent."