Trump condemned former prime minister Theresa May's "foolish" attempts to get a Brexit deal and suggested Britain leave without one - an outcome businesses warn could be disastrous.
"What we don't do traditionally as loving allies and friends, what we don't do traditionally, is get involved in each other's election campaigns", Johnson, 55, advised LBC radio.
Britain's politicians are bracing for a two-day visit to London by U.S. President Donald Trump and wondering how the trip may affect the most volatile and toxic British election in decades, one likely to shape the country for generations. But with his own election fast approaching, Johnson wants to keep his distance.
A USA official told reporters in Washington that Trump "likes Prime Minister Boris Johnson personally, but he's absolutely cognizant of not wading into other countries' elections".
Senior Conservative leaders are wary of the fact that Trump could upset the campaign by wading into the elections during the North Atlantic Treaty Organisation meeting scheduled in London next week.
Trump has already waded into the election, saying in October left-wing opposition leader Labour Party, Jeremy Corbyn, would be "so bad" for Britain and that Johnson should do a pact with Brexit Party leader Nigel Farage.
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"Many of those preparations will be extremely valuable as we come out of European Union arrangements anyway", he told reporters.
Trump, who got back to the United States on Friday from a whirlwind trip to Afghanistan, arrives in London on Monday night for two days of meetings with North Atlantic Treaty Organisation leaders gathered for the summit.
Johnson said President Trump "brings his style" to the way he handles foreign policies or immigration issues, and his supporters like that style, and that is the reason they voted for him and will do so again next year.
Corbyn has used Trump's praise of Johnson as one of his focal messages to attack the Conservatives in his campaign, saying they would sell off parts of the much-loved state-run National Health Service to the USA businesses after Brexit if they win the election.
However, when asked whether the lack of a meeting with the Prime Minister was related to Mr Johnson's urge for the President to steer clear of United Kingdom politics, the senior USA official said "that's something we're still working on".
However, he declined to say how many children he had, saying that he would not "put them on the pitch" ahead of the December 12 election.