Time Inc. announced a new top management team today that will be headed by J. Richard Munro as chief executive officer and president.

The new appointments will take effect Oct. 1 when Chairman Andrew Heiskell and President James R. Shepley resign. Heiskell will have reached the mandatory retirement age of 65, but Shepley, 63, will move to Washington to devote his energies to The Washington Star, which Time bought in 1978.

In addition to keeping his title as chairman of the Washington Star, Shepley will remain chairman of the executive committee of Time Inc.'s board of directors.

Heiskell and Shepley's plans to step down were made public long ago and led to considerable speculation about who would head the publishing and forest products corporation in the 1980s. Time Inc.'s 1979 revenues were $2.5 billion, about $1.1 billion from publishing, $925 million from forest products and the remainder from other activities.

The three men who were named to top positions today have been the central figures in speculation over the past two years, but as recently as Time's annual stockholders meeting last month, the directors had refused to announce the new leadership.

Munro, 49, will have Ralph P. Davidson as chairman of the board and Clifford J. Grum as executive vice president reporting to him.

Munro began his Time career in 1957 with the circulation department of Time magazine. He has been publisher of Sports Illustrated, president of Time's chain of weekly newspapers in the Chicago suburbs, and last August was named to the newly created position of executive vice president. At the same time, he became chairman of the company's new corporate operations office of which davidson and Grum were members.

Davidson, 52, "will represent Time Inc. to its various public, both in the United States and abroad," the board of directors announced. This will include supervising Time's relations with the federal government and with foreign and local governments, and heading the company's programs of social responsibility.

Davidson has been a vice president. He joined the company in 1954 as a Life magazine retail representative and has held many positions, including publisher of Time magazine.

Grum, 45, will become executive vice president and will oversee the company's five group vice presidents. He joined the company in 1973 when Time acquired Temple Industries Inc. of which he was vice president.

Munro, Davidson and Grum are all Time board members.

Arthur Temple, 60, who headed Temple Industries, remains vice chairman of Time Inc.

In other management changes, the board of directors announced that Arthur W. Keylor, group vice president for magazines, who had been mentioned as a future top executive, is taking early retirement at the end of the year.Heiskell said Keylor informed him two years ago of his intention to step down at age 60.

Kelso F. Sutton, 41, who has been a vice president of Time Inc. and publisher of Sports Illustrated, will succeed Keylor.

Philip G. Howlett, 52, will become publisher of Sports Illustrated.