Philip McBride Johnson announced yesterday that he will resign as chairman of the Commodities Futures Trading Commission by May 1.

Johnson, a former top commodity lawyer in the Chicago firm Kirkland and Ellis, told President Reagan that he was resigning for "personal reasons." His resignation came as a surprise to most of his colleagues and staff members at the CFTC, which regulates trading of futures contracts in such diverse products as grains, fuel oil and Treasury bonds.

Johnson announced his resignation at a meeting of the securities and commodities lawyers segment of the American Bar Association in Puerto Rico. He told reporters the resignation was not triggered by dissatisfaction with the job. "It was just time," he said. Johnson's friends said they think he will return to law practice, but in New York or Washington rather than in Chicago.

During Johnson's tenure the CFTC resolved a long-standing territorial dispute with the Securities and Exchange Commission and won a difficult reauthorization fight in Congress.

CFTC sources said they think Johnson feels he has accomplished what he set out to do.

Johnson is the third of the five CFTC commissioners to resign in recent weeks. Commissioner David Gartner left in November to join the Washington law firm Peterson, Engberg and Peterson and Commissioner James Stone will leave at the end of this month to start a new insurance company.