The Prime Minister insisted it was "never right to be abusive or rude" to a police officer and said the judge had "made very clear his verdict".
Mr Justice Mitting decided that the former chief whip did call Pc Toby Rowland a "pleb" because the officer did not have the "wit, imagination or inclination" to invent it.
Asked about the outcome of the case after he made a keynote speech in Staffordshire on immigration, Mr Cameron said: "On the issue of Andrew Mitchell, I mean, let me be clear - it is never right to be abusive or rude to a police officer. I think that is extremely important.
"But, look, we've had a court case now. That's how we do things in this country.
"The judge has made very clear his verdict and I think everyone should accept that verdict and move on."
Mr Justice Mitting said he had reached the "firm conclusion" that the 58-year-old MP used the "politically-toxic" word "pleb" in Downing Street in September 2012 when he was not allowed to cycle through the main vehicle gates.
Mr Mitchell, who resigned as whip a month after the altercation, vehemently denied during his two-week libel action against News Group Newspapers (NGN) that he had said: "Best you learn your f****** place - you don't run this f****** government - you're f****** plebs."
He said he would never call a policeman a pleb "let alone a f****** pleb", although he agreed he muttered audibly under his breath "I thought you lot were supposed to f****** help us" - but not directed at the officer.
The Conservative MP, who now faces a hefty legal bill after losing the case, said he was "bitterly disappointed" with the ruling.
A senior police constable who was on duty in Downing Street at the time of the row said he feels sorry for Mr Mitchell.
Ian Richardson, who recently retired after 30 years in the Metropolitan Police, said it was a "nonsense incident" and criticised Scotland Yard's handling of the row.
He said no-one should have lost their job over what happened at the gates of Downing Street and accused the Police Federation of "jumping on the bandwagon" and using the incident as a "hook to hang their grievances on".
Mr Richardson, who heard part of the exchange and gave evidence in the High Court case, told the BBC Radio 4 Today programme that the Met should have drawn a line under the incident, which had been leaked to the press.
"I said in my evidence in chief last week that it was a nonsense incident that shouldn't have cost Mr Mitchell his job, certainly shouldn't have cost the officers who were whistleblowers or - whatever your opinion of them is, whether they were leakers or whistleblowers - it shouldn't have cost them their jobs.
"But here we are, with that.
"Mr Mitchell lost his sense of humour. Who hasn't lost their sense of humour at work?"
Asked if he felt sorry for Mr Mitchell, Mr Richardson said "I very much do", but also expressed sympathy for the police officers who lost their jobs as a result of the investigations into how details of the incident were leaked.
"It was all totally unnecessary," he said. "I do feel sorry for Mr Mitchell, yes."
Mr Richardson said he had written to his superior officer setting out what had happened in the hope that the Met could have put the matter "into some kind of reasonable perspective to draw a line under it there".
But he said: "They were very interested in the leak and to find out where the leak was, but I was told that who said what to whom at the gate was of no interest to the organisation. The Commissioner had decided that a line would be drawn under it there."
Mr Richardson said: "It could have been said that it is a minor incident that has be leaked to the newspapers and let's all move on.
"But of course we didn't move on and we ended up all at the High Court last week and this week.
"It's a great shame and at what cost to so many people."
Mr Richardson said the Police Federation needs "serious reform", adding: "I think they need to more clearly represent their members. This was not a hook to hang their grievances on."
He also criticised Met Commissioner Sir Bernard Hogan-Howe's handling of the affair: "I was a lowly Pc. I think he could have done more at the time, but who am I to say that."
Former Tory MP Michael Brown said Mr Mitchell would not have got into trouble if he had been using a ministerial car rather than a bicycle.
Mr Brown, a friend of the former minister, said "there's room for Andrew Mitchell to rebuild his life" and he should now concentrate on retaining his seat at the general election.
He told Today: "I think that Andrew has simply got to make sure that in 23 weeks today, May 8, he has received the judgment not of the High Court but of the voters of Sutton Coldfield.
"I'm confident, having been to speak for him several years ago, that he is held in very high regard there."
He added: "Andrew should not have been rude to the police. Whether or not he used the word 'pleb', he should never have done that."