London — Peter, sixth Baron Carrington, is about to take charge of the North Atlantic Treaty Organization as secretary-general. Very soon the Western alliance will be in the care of a professional politician who likes to operate under the colors of an aristocratic English amateur.
Lord Carrington's appointment, which follows the long reign in Brussels of Joseph Luns from the Netherlands, is his first official job since his resignation as British foreign secretary at the height of last year's Falkland Islands war.
Apart from minor hesitations by Spain, which worries about the implications for Gibraltar of having an Englishman in charge of NATO, Car-rington's endorsement by the member nations has been wholehearted.
Carrington, the nearest thing to a Conservative grandee that British politics can produce, is a serious apostle of alliance unity. He is convinced that, far from being in disarray, NATO remains a strong, resilient shield against communist aggression.
Criticism of him tends to arise from the off-hand, hokey style he adopts when cornered for an interview.
After his Falklands resignation, right-wing Tories accused him of being a once-over-lightly politician who failed to spot Argentine intentions in the South Atlantic until it was too late.
At the time, Carrington was focusing hard on the crisis in Lebanon, and Gen. Leopoldo Fortunato Galtieri's posturing was on the point of boring him. Having presided over a great department of state that had, in the Whitehall idiom, ''got it wrong,'' he decided to bow out gracefully.
Months later, an official inquiry exonerated him from blame over the Falklands invasion, but Tory diehards continued to complain about his free-wheeling, aristocratic way of conducting official business.
Margaret Thatcher, the prime minister he served for three years as foreign secretary, thought differently. She is believed to have had a role in promoting him as the next ''permanent head'' of NATO.
Carrington has assurance that wealth and perfect pedigree help to supply. He has served six prime ministers, starting with Churchill.
Since succeeding to the barony at age 18, Carrington has held political jobs ranging from eradication of rabbits (Ministry of Agriculture) to putting up political defenses against the Soviet Union.
Officials who served him when he was president of the 10-nation European Council of Ministers appreciated his refusal to tolerate long-winded speeches and welcomed his way of dispatching business.
A former adviser characterizes his style as combining ''grand but necessary simplification of complex issues'' with polite refusal to suffer fools gladly. Although he is no economist, he has when necessary read himself into the details of a complex economic brief with long hours of study.
He is forever the aristocrat.
Critics say one of his faults is to seem less serious than he is. NATO, in his view, has always been assumed to be in trouble, and each difficulty seems to be worse than the last. But Carrington says that, in reality, the alliance is in fairly good shape. Former Foreign Office associates expect him to devote his formidable energies to the job but to pretend it is all an amusing distraction.
Carrington's greatest achievement has probably been the negotiation that enabled Rhodesia to become independent Zimbabwe. Mrs. Thatcher at first found it hard to see how she could get on terms with black nationalist leaders. But Carrington cajoled and gently bullied Robert Mugabe and Joshua Nkomo toward an agreement on an independence constitution.
The resulting triumph won him the profound distrust of the right wing of the Conservative Party. When he struck trouble in the South Atlantic, his liberal record on Zimbabwe was a handicap. Many Tory backbenchers were glad to see him go.
NATO has good reason to welcome him. As foreign secretary, he was a tireless traveler, and alliance diplomats expect him to work hard to smooth out differences between Europe and the United States.
At the Foreign Office, his relations with Presidents Jimmy Carter and Ronald Reagan were equally good.
A former Foreign Office colleague says, ''Peter likes Americans. . . . He understands the problems of dealing with US officials. He also readily commands the confidence of Europeans.''
This official commends his seemingly offhand style, which is designed to persuade people that even serious problems should never be allowed to be boring.
''Peter is a nobleman with many acres in Bucking-hamshire, an entrenched position on the radical wing of the Tory party, a lot of money, ready wit, a commitment to alliance unity, and 32 years of political experience. It's a formidable mixture, and NATO will get the full benefit of it.''