After a stormy career in British politics that spanned nearly three decades, Roy Jenkins became a man without a country in early January.

He stepped down as deputy leader of the Labor Party last year, packed his bags and left for the continent after Prime Minister James Callaghan suggested a job, Jenkins, says, he "could not refuse."

In assuming the four-year presidency of the European Economic Community's 13-man executive Commission, Jenkins, 56, vowed to forsake national prejudice and revive Europe's torpid quest for unity.

As the new "Mr. Europe," he has spent much of his first three months in office trying to build a base as power broker among the Common Market's nine governments and to forge closer working ties with the Carter administration.

"I was very glad that Vice President Mondale chose Brussels to begin his visit of allied capitals," said Jenkins in an interview here."I received a strong impression that the new administration wants to work seriously with the community.

"Our thoughts on broad, world economic questions march reasonably well in step, both in relations with developing countries and the need to balance economic expansion with protection against inflation," he continued.

A devout believer in the Atlantic Alliance, Jenkins envisions "molding a better partnership, closer to equality than in the past, with America" as one of his chief goals. "An effective, united community makes the world a less lonely place for the United States," he said.

Jenkins expects to visit Washington and confer with President Carter in late April. The most likely topics on their agenda include the multilateral trade talks in Geneva, the West's approach toward developing countries' demands at the conference on international economic cooperation, and the Western world's economic summit in London in early May.

So far, Jenkins has not been invited to the London session, a slight that has irked him and the EEC's smaller countries.

France has blocked EEC representation, embodied by Jenkins, on the ground that the conference would become too unwieldy. Seven countries - the United States, Canada, Britain, Japan, Italy, West Germany and France - have already been invited.

The smaller Common Market countries have protested that the exclusion of an EEC delegation surrenders their fate to the bigger community nations and makes a mockery of the organization's aspirations toward unity.

Beyond the current dispute, Jenkins believes that prospective membership for Greece, Spain and Portugal poses a crisis for the community.

"There is no question of banging the door shut on these nascent democracies - indeed, there is a political imperative to give them a satisfactory answer - but we must consider the economic difficulties or else a 12-nation community could drift back to being just a free-trade zone," he said.

Jenkins favors massive infustions of aid to help the three countries adapt their economies to the brisk competion they would face on full entry.

Negotiations with Greece are in full swing, Spain plans to not apply until after this summer's parliamentary elections. Portugal expects to submit its membership request at the end of this month.

"Elarging to 12 countries will also require drastic changes in Common Market institutions," notes Jenkins. At present, EEC ministers must reach decisions through unanimous viting - "an unworkable system when there will be 12 members."

The imminent expansion has also focused Jenkins' attention on the working methods of the EEC Commission, where 10,000 "Eurocrats" plod through their daily tasks in six different languages. Translation work alone occupies a third of the staff.

Despite the bleak outlook for European unity in an era of growing economic and political turmoil around the continent, Jenkins sees one bright spot in the direct elections for the European Parliament, scheduled for May 1978.

Jenkins feels that the Parliament must be strengthened if the community is to evolve in a democratic fashion. Virtually impotent today, the 135-seat chamber could bolster its stature next year if such notable politicians as West Germany's Willy Brandt and French Socialist leader Francois Mitterrand fulfill their promises to run for seats in it.

The son of a Welsh parliamentarian, Jenkins won his own seat in the House of Commons from central Southwark in 1948.

From there, he rose steadily through the ranks of the Labor Party.

He became deputy leader of the party in 1970, and nearly provoked a party split in his bitter clashes with Prime Minister Harold Wilson over British membership in the community.

When Britain's turn to appoint the president of the 13-member Commission came up last year, Callaghan promptly selected Jenkins to succeed France's Francois-Xavier Ortoli.

The choice was applauded by other EEC government leaders, who praised Jenkins' fervent support of the European cause.

Jenkins has encountered some organizational problems in his first months, but most observers are still hopeful that, in time, he will lend the commission a political dimension it has sorely lacked.