Johnson, who was on trial for almost four weeks, will be sentenced at a later date.
A USA jury has found a former HSBC executive from the United Kingdom guilty of defrauding client Cairn Energy in a 2011 currency trade. She said the office would continue to prosecute those who "undermine public confidence in the operation of the financial markets by engaging in fraudulent schemes".
Johnson, 51, was accused of exploiting confidential information from Cairn Energy in 2011 to "front run" the oil and gas firm in the currency markets, thus making an $8m profit for the bank at the expense of its client. HSBC declined to comment. HSBC spokesman Robert Sherman said before the trial that Johnson left HSBC earlier this year, and Scott left in 2014.
In the front-running scam, Johnson and a co-worker, Stuart Scott, bought British pounds from a bank's account ahead of a giant Cairn order - those falsely inflating the price of the currency and the fees paid by Cairn.
A witness for the prosecution said nearly two thirds of sterling's trading on a major platform were dominated by HSBC traders in the five minutes before the deal.
The prosecutors' case revolved around recorded telephone calls, including tapes in which the higher price for the pound was blamed on a Russian buyer.
Johnson's lawyer, John Wing, told reporters as he left the courtroom: "They've convicted an innocent man". He said that Johnson and his colleagues had tried to get a fair price for Cairn and even given Cairn a rebate. The charges marked the first in the US Justice Department's three-year investigation into foreign currency rigging.
During the trial, lawyers for Mr. Johnson argued that he had been acting in the client's best interest, that prosecutors were trying to criminalize normal currency trading activity and had twisted their client's words to create the illusion of a conspiracy.
In September, the US Federal Reserve fined the bank $175m in over "unsound" practices in its foreign exchange business.