U.N. Ambassador Andrew Young's last official trip, a two-week trade mission to Africa, brought $1.5 billion in new business to the United States, Young said yesterday. He called it an unqualified success.

Young's resignation after nearly three years in his post becomes effective today.

His tour of 10 African nations with 22 American businessmen generated "serious business" in eight of the countries, Young said. Deals made, pending formalization , or in preparation total $1.5 billion now, will be $1.75 billion by the end of the year and are likely to pass $2.5 billion within two years, he said.

"President Carter was very well pleased," Young told reporters after briefing the chief executive. Part of the purpose of the trip, Young continued, was to assure African leaders of continuity in U.S. policy toward their region after Young's departure.

"Our long term self-interest . . . essentially coincides with Africa's self-interest," he said. The United States is dependent on Africa for nearly 40 percent of its improted oil and will spend $12 billion more in Africa than Africa spends here this year, Young continued.

"You've got basically a battle for markets going on," he said. "A return to 'fortress America' is not a realistic possibility." Young said he planned to continue "trying to build a relationship with the rest of the word" when he returns to Atlanta as a private citizen.

"This is the critical issue of the 1980s," he said. "I don't think we're in nearly as much danger from the Soviet Union as we are from our economic competitors." He said he would work in "kind of a private consulting business, but nonprofit." In an interview, he said he might call it "Young Ideas Inc." -- one of his old election campaign slogans.

"Not particularly" does he want to return to Capitol Hill as a member of Congress, looking on his African trip as "a kind of fulfillment" of goals he had retained from his days as a representative from Georgia through his ambassadorship.

"I really enjoyed it all," he said of his controversial U.N. work. "There was never a time when I got discouraged or frustrated. I always knew where things were going and where I wanted them to go, and that's pretty much where they went. It was all a high point."

Young told a Washington Post interviewer before leaving Senegal that he had "already started" stumping for the reelection of President Carter, whom he lauded to his African audiences as the greatest president for blacks in U.S. history.

He said he would complete a book on his civil rights battles of the 1960s and would visit the Middle East -- including Israel and several Arab nations -- by mid-November at the urging of the American Jewish community.

"I really don't believe there are bad feelings between blacks and Jews," Young said, referring to the recent fuss over his and other black leaders' meetings with members of the Palestine Liberation Organization. "I mean, black folks are demonstrably less anti-Semitic than white folks."

In Washington, Young refused to discuss any political questions, but he spoke openly in the interview of his effort to enlist African leaders in promoting an Arab-Israeli dialogue. Continued Mideast turbulence, he told them, hurts African development through oil price hikes and shortages.

But Cameroon's Ahmadou Ahidjo "didn't want anything to do with it," Young said of the dialogue, while Nigerian president-elect Shehu Shagari was noncommittal.Only Liberian President William Tolbert, chairman of the Organization for African Unity, was receptive. "Any followup would be largely through President Tolbert," Young said.

A Mideast settlement would particularly help eastern Africa, he continued. "The Saudi Arabians would get involved in East Africa" with their oil wealth then, he said. "The Saudis tend to see East Africa as their security sphere."

The departing ambassador said he was optimistic that there would be no civil war in Zimbabwe-Rhodesia, as long as the two wings of the guerrilla movement come to power jointly. "Once one of them loses the elections, it will be difficult for them to fight without support" from other African nations. "Everybody will gradually drift to the winner," he said.

With Young at the Washington press conference were John Moore, president and chairman of the U.S. Export-Import Bank, and Bruce Llewellyn, chief of the Overseas Private Investment Corp. who accompanied Young on the trade mission.

Moore said some of the new trade agreements involved wood alcohol production in Liberia, a fertilizer plant in Nigeria, the sale of a Boeing 747 jetliner to Cameroon, and banking arrangements to help small businessmen in Kenya and Cameron.

Llewellyn, saying that $1 billion in sales overseas means 30,000 jobs at home, declared, "We raised the flag in Africa that we are there to do business."