Johnson said adding a third runway to the international airport would be "a giant step backward for London" and told BBC London capacity at Gatwick airport could be focused on instead.
The third runway was proposed as a means of increasing international travel in southwest England in a recent report by the British government. The idea has not been well received by residents in the Heathrow area who predicted round-the-clock aircraft noise and possible health risks.
Proponents of the expansion said Heathrow, which already handles more than 69 million passengers annually, is vital to Britain's economy and would be more competitive in the overall European market if the new runway was built.
Johnson, however, was not taking the local opposition lightly.
"I think my party, the Conservatives, would be utterly nuts to go into the next election with the possibility of a Heathrow third runway on the table," he told the BBC.