In what may be one of the most extraordinary diplomatic appointments in years, the man who for 15 years represented Rhodesia's white-minority government in the United States has returned as chief envoy of the new Zimbabwe government of Prime Minister Robert Mugabe.

Only a few months ago, Kenneth Towsey, 60, was still going around town and muttering dark warnings about Mugabe and other black nationalist leaders and the Marxist threat they represented throughout southern Africa.

With considerable skill and aplomb, he mustered conservative forces on Capitol Hill in repeated attempts to lift economic sanctions against the government of Ian Smith.

Nowadays, as charge d'affaires for Zimbabwe, Towsey refers to "the prime minister" with an air of respect that seems to obliterate the past.

Diplomatic observers here suggested that Mugabe displayed exceptional shrewdness in appointing Towsey as his representative in Washington. Towsey's friends include many influential conservative figures on Capitol Hill such as Sen. Jesse Helms (R-N.C.), Sen. Harry F. Byrd (I-Va.), Rep. Robert Bauman (R-Md.) and Rep. Edward Derwinski (R-Ill.).

It was Towsey's credibility with the conservatives that ensured virtually no opposition to $30 million in aid to Zimbabwe included in the administration's foreign aid bill when it came up for a vote last week in Congress.

But the appointment of Towsey also seems to follow the general thrust of Mugabe's policies since he became the leader of an independent Zimbabwe last April.

Despite bitterness generated by the long Rhodesian civil war, Mugabe has largely dropped Marxist rhetoric in forming a government representative of various views and both races. While underscoring his intention to correct past political and economic injustices suffered by blacks, he said he would do so without creating new injustices against the white minority.

Mugabe has also signaled willingness to cooperate with the West. Towsey disclosed in an interview that unlike the marxist government of neighboring African states of Angola and Mozambique, the new Zimbabwe government has applied for membership in the International Monetary Fund and expects to become a member later this month.

"For the first time in many years I find myself doing constructive things for my country," Towsey said about his new job.

"In the past, my role was somewhat negative," he continued without elaborating oh his years-long lobbying efforts on Capitol Hill to prevent American backing for black-majority rule.

"It's been encouraging to me in this role to find responses here of wanting to be helpful to the new Zimbabwe, of having some confidence in it largely established by the very constructive and statesmanlike position that the prime minister has taken. These attitudes have extended to conservative members of Congress.

"I have found that conservative members of Congress have been willing to accept what I have been able to explain to them."

Towsey, an Oxford-educated Briton who migrated to Rhodesia after World War II, talked about vast reconstruction tasks to repair war damage in Zimbabwe, rebuild schools and hospitals and resettle tens of thousands of displaced persons.

Apart from securing cooperation of the U.S. government, he saw his main task as promoting American private investments in Zimbabwe.

His long tenure as head of the Information Office of the "old regime" of white prime minister Ian Smith (which was never extended diplomatic recognition) is now behind him. "I'm a career civil servant. As far as I am concerned there's a professional code and you serve any government that happens to be in office."

But he acknowledges, a trifle grudgingly, that by agreeing to serve in the Mugabe administration he was betting on the prime minister and his program."We have a chance to succeed," he said, adding that this is a feeling shared by "many white Rhodesians particularly those in senior administrative positions." "

Towsey's private life is likely to undergo a drastic change over the coming months. For years a pariah in the diplomatic community when he represented a breakaway British colony, he is now the envoy of a popularly elected black prime minister in one of the key diplomatic capitals.

Was he likely to eventually become Mugabe's first ambassador here? He was serving, he said, "at the prime minister's pleasure."