'M'ALINEO TAU IS leaving Washington this week.

There have been no elaborate farewell parties, no dinners or dances. Mrs. Tau will simply pack her bags, go out to the airport and get on a plane. I myself only learned of her plans the other day, when I ran into her in the produce department of the grocery store. "This is the first time they've had nice tomatoes here in weeks," she said earnestly. "By the way, I'm leaving. I've been reassigned to London."

Mrs. Tau is not well-known to the general American public. She is not even especially familiar to the foreign policy community in which she works. But in her five years as ambassador to the United States from Lesotho, a tiny mountain kingdom entirely surrounded by South Africa, she has become a hero at home and has stirred admiration and respect in a small circle here as one of the most effective people in the gigantic Washington diplomatic corps. She has learned how to operate in the American system better than some of us ever will, and in her own modest way she has helped educate some key people about Africa.

"She had a wide range of friends and contacts," said Sen. Nancy Kassebaum (R- Kan.), chairman of the Senate Africa subcommittee. "She gave Lesotho a representation far beyound its size and borders."

Mrs. Tau comes from a very humble background. Her father is a bus driver in Soweto, the sprawling black ghetto outside of Johannesburg. Her rise in government service has been nothing short of meteoric: Her last job before she was named ambassador here was as librarian of the National University of Lesotho, and she is quick to admit that she got where she is through political connections.

Lesotho is not the easiest -- or, in some respects, the most desirable -- place to represent. Since independence in 1966 (it used to be the British protectorate of Basutoland), it has, unlike South Africa, been ruled by its black, primarily Sotho-speaking majority. But its system is not exactly democratic. The prime minister, Chief Leabua Jonathan, is a mostly benign despot who has been known to annul elections when he does not like the results; the banned leftist opposition party has reemerged in recent years as a guerrilla group, the Lesotho Liberation Army (LLA).

Indeed, some would say that Lesotho is a nation in name only.

Its borders are totally porous and, therefore, easily controlled by South Africa, whose agents, public and secret, are a permanent fixture in Lesotho's forlorn capital of Maseru. (One of South Africa's most controversial actions in the region of recent years was a pre-dawn raid on Maseru in December 1982, when several suspected guerrillas of the African National Congress, as well as a number of South African refugees and Lesotho citizens, were killed.)

It is impossible to fly to Lesotho without going over South Africa territory, and most travelers from elsewhere in Africa must first pass through Jan Smuts Airport in Johannesburg. Without South African food and cooperation, the people of Lesotho would starve. Lesotho's main source of income is the money sent home by its men who work in South African gold mines. Although the LLA is ideologically objectionable to South Africa, Pretoria is believed to be its primary supporter; that is one more way to demonstrate who is boss in the region.

Understandably, the Lesotho Embassy here is not a very grand place. The chancery, where Mrs. Tau has presided over a staff of five, is on the third story of a small office building near Dupont Circle, two flights up from a pizza parlor. The last time I was there it was still using the threadbare green carpeting left behind by a prior tenant, and the map in its waiting room was a faded old Rand McNally edition on which several African countries still had their colonial names. The telephones often went unanswered for a lack of personnel.

Mrs. Tau's first task on arrival, while the Carter administration was still in power, was to try to patch things up with the State Department. After years of behaving like a solidly pro-Western mini-state, Lesotho, in a burst of nonalignment, had just established relations with the Soviet Union, no doubt hoping to be able to play the superpowers off against each other to its own benefit.

But Washington no less than Moscow, under Democrats no less than Republicans, prefers that Third World countries choose sides in the great global power struggle, and so Lesotho was in the deep freeze. Mrs. Tau found it very difficult at the time to get appointments at the Agency for International Development (AID).

Slowly she mended the fences, making it clear that she above all, an alumna of Pratt Institute in New York, appreciated the American way. Before long, in fact, she had found friends on Capitol Hill and had developed a special technique for getting more American assistance for Lesotho.

'M'Alineo Tau went on the prayer-breakfast circuit, recruiting one church after another to extend private help to a place they had never heard of before. It all began when she was invited to a prayer meeting at the home of a congressman from Washington state, whom she had met at a conference. She found the people there surprisingly interested to learn about Lesotho, and they in turn put her in touch with others. By January 1981, in the last days of the Carter presidency, she found herself addressing a high-powered prayer breakfast in Baltimore, which was also attended by Carter and a number of black congressmen.

"Since then, I've been going around with the AME (African Methodist Episcopal) Church," Mrs. Tau explains proudly. During one weekend trip to Oklahoma City, in fact, she persuaded a number of individual congregations to make it possible for several small villages in her country to have new wells (and she made a local bishop an honorary consul of Lesotho in the bargain). Later she did the same in Buffalo and other cities.

Mrs. Tau's church contacts helped her find an American company that makes boreholes and other necessary equipment for the wells, and she persuaded businessmen she met to pay the shipping costs. For the Embassy of Lesotho, there was no exchange of cash, no bureaucracy to deal with, and unlike many government-to-government assistance programs, little prospect of "loss" -- corruption -- along the way. She found a way to get American aid without American AID.

Mrs. Tau lived not very ostentatiously in the Maryland suburbs. In her spare time, she enrolled in courses in "information science" at the University of Maryland, feeling this might help her future job prospects if her diplomatic career should be cut short at some point. When her 21-year-old daughter drowned accidentally, she endured the tragedy with dignity and took the body home to be buried in traditional African fashion.

What Mrs. Tau did not do is what many ambassadors from African and other Third World countries to the United States and the United Natins spend a great deal of their time doing: complaining to each other about how little interest Americans have in them and their nations' problems. She simply demonstrated to Americans why they should care.

It is not that Mrs. Tau did not have her polemical side. On the contrary, she could deliver a stem-winding attack on western indifference toward African development or on U.S. policy toward South Africa as well as anyone. Sometimes this made people resentful, but all she really wanted to do was make them feel uncomfortable, and stir them to action.

She never let protocol get in the way of communication. If it was 8 a.m. or 8 p.m. and she had something on her mind, she picked up the phone and called. Her manner was open and unassuming. Yet it seemed that she took everything very seriously. Encountered on the telephone, in the street -- or at the grocery store -- and asked how she was, Mrs. Tau would characteristically respond, "As well as can be imagined."

I first met Mrs. Tau at a conference of Africans and Americans held in Africa. She was holding forth on her favorite subject of "women in development," on the need for Africans and the outside world to rethink, and come to a new appreciation of, the role that women could play in developing societies. She was several years ahead of her time. Mrs. Tau was at the international conferences on women in Nairobi in recent weeks; the participants there could have used some of her earlier remarks as their text.

While working on a book about Africa for a general American audience, I sought to find out more about the life and the role of the African diplomatic community in Washington. No one, it turned out, knew the subject better than this representative of one of Africa's smallest countries.

She spoke to me of her disenchantment with some of her fellow African ambassadors who, alienated and ineffective in Washington, pathetically stayed at home, drinking the ample supplies of liquor their governments provided and slowly sinking into alcoholism. And she gave me a short list of the other ambassadors whom she thought it would be most worthwhile to interview. She was invariably right: When I spoke with someone she had warned me about, I found that I was wasting my time.

If I were to try to draw up the list of the most valuable and interesting people I've known in Washington in the past 16 years, 'M'Alineo Tau, would have to be near the top.

Now she is off to London. The bus driver's daughter will be ambassador to the Court of St. James. Obviously, somebody in Maseru recognizes her talents, and I hope the English will, too.

Goodbye, Mrs. Tau.