Johnson has admitted he and designer Thomas Heatherwick met with Apple executives in San Francisco in February 2013 to discuss corporate sponsorship of the Garden Bridge, at least a week before other architects were invited by Transport for London to tender for the design contract. Heatherwick went on to win the contract in March.
The Mayor, who is also chairman of TfL, previously refused to say who he had met with in the city and had listed the trip as private in his diary. The Architect's Journal uncovered the details of the trip - including Heatherwick's presence at the meeting - as part of an ongoing investigation, using the Freedom of Information Act.
In a statement the mayor's office said the meeting had no bearing on TfL's procurement process.
A spokesperson said: "The mayor met with Apple in 2013 to discuss a number of investment opportunities in London. Thomas Heatherwick was also in California to meet a separate commitment with Apple. Given that he had already expressed interest in creating a Garden Bridge, the mayor invited him to join the meeting and outline his ideas.
"That meeting had no bearing on the procurement process led by Transport for London for the design of the Garden Bridge, which was open, fair and transparent."
The mayor's office added that discussions with Apple about potential investments in London were still ongoing and Johnson did not wish to prejudice those conversations.
Procurement for the bridge design contract has faced scrutiny for some time following accusations that Heatherwick's winning of the design bid was a done deal.
It was also revealed earlier in January that Chancellor George Osborne offered Johnson £30m of Department for Transport funding for the bridge, without the department's oversight. The National Audit Office, which audits government finances, looked into the case for Treasury funding of the bridge last year. It wrote a letter to MPs suggesting there did not appear to be "a compelling value for money case" for funding the bridge, and implied the department may not have given the funding had normal process been followed.