Francois Maurice Marie Mitterrand, the 64-year-old Socialist elected as France's 21st president yesterday, has reshaped French politics with his unexpectedly easy victory over his rightist opponents and will now attempt to reshape France's role in the world.

The attempt on the international scene may be even more difficult for this highly intellectual and tenacious politican than was his 16-year active quest for the Elysee Palace. His chances depend to a great extent, ironically, on the attitudes of two forces that are deeply antagonistic to his brand of socialism, and to each other -- his former ally on the left, the French Communist Party, and the Reagan administration.

Mitterrand had to overcome an image as a man of the past and the divisions that plague the large and ideologically unwieldly Socialist Party that he personally reconstructed during the last decade to defeat incumbent Valery Giscard d'Estaing.

To get what he wants internationally, he will now have to overcome the sense of loss felt in Washington, where the successive U.S. administrations saw Giscard as the leader in Europe most willing, and able, to work with the United States to protect Western interests when they came under challenge in the Third World. Mitterrand is not likely to aspire to, nor to gain, such accolades from the Reagan administration.

But his views toward the Middle East and toward the Soviet leadership are likely to be more in turn with those of the Reagan White House than were Giscard's. Although Mitterrand will differ on many issues, his views may hold the key to refashioning a working alliance between Washington and Paris. These appear to be some of the results of the French vote:

West European countries emphasizing Palestinian self-determination as the most important factor in an Arab-Israeli peace settlement have lost their leader. Mitterrand, who publicly endorsed the Camp David peace agreement, is much friendlier than was Giscard to Isreal, and may well review the large-scale French arms sales to Arab nations.

Soviet leader Leonid Brezhnev has lost the special relationship that he had developed with Giscard, who has said that Brezhnev personally was a good man ocassionally betrayed by evil aides. Mitterrand indicated in the campaign that he will be much tougher on Brezhnev and on the Soviet disarmament schemes and peace overtures that Giscard took largely at face value.

The Paris-Bonn political axis that Giscard and West German Chancellor Helmut Schmidt constructed has lost its center. Mitterrand shows no sign of sharing Giscard's openly expressed admiration for West Germany as a model for France, and the left wing of Schmidt's Social Democratic Party is likely to be emboldened by the Mitterrand victory to make life even more difficult for the beleaguered Schmidt.

Willy Brandt -- not Schmidt -- is Mitterrand's most important German friend. Wandering in what seemed to be the political wilderness of the Socialist International since he challenged and lost to Gen. Charles de Gualle in the 1965 French presidential election, Mitterrand has spent his time with Spain's young Socialist chief Felipe Gonzalez, with Portugal's Mario Soares and even with the Italian Communist Party leader Enrico Berlinguer rather than with Britain's Margaret Thatcher or the Social Democrats of Scandinavia, whom Mitterrand appears to distrust.

Because of these friendships and because of the task he faces at home, Mitterrand's victory gives French politics a Latin European cast that Giscard abhorred. Mitterrand will now attempt to move the economy to the left while reducing the clout of the French Communist Party even more than he has in this campaign.

Giscard ruled in a contradictory fashion, saying that he was changing nothing in the fiercely independent foreign policy De Gaulle had created while changing a good deal and moving quietly toward Washington. Internally, Giscard promised and boasted of change and reform that he did not deliver.

Mitterrand faces a situation that could bring him into an equally contradictory series of choices. Unless forced into it by hostility from Washington, he will probably not want to disturb the close Atlantic relationship shaped by Giscard but he will probably have to give outward appearances of reestablishing a Gaullist distance.

At hoame, Mitterrand will have to promote social and economic change to satisfy the electorate that has brought him to power, but he will probably try to minimize the true extent of that change in order to prevent a flight of capital and other economic turmoil that could destroy his promise of renewed, and more justly distributed prosperity.

Mitterrand has said that he wants for France a model of socialism that is based neither on Eastern Europe nor on Scandinavia. His long and intensely controversial political career has been marked more by an ambition to get to the top than by a clear commitment to a specific ideology, and he has around him a coterie of advisers with differing viewpoints and policies from which to choose when he sits down to form a government.

In one sense, he comes to power much as Ronald Reagan did in the United States, with his personality and perserverance being more important to the victory than were party structure or specific issues. While he owns much of his victory to Communist voters, he owes nothing to the Communist leaders who broke with him savagely in 1978 when he made a sincere attempt to forge a Union of the Left to win that year's legislative elections.

The Communist betrayal of that effort positioned Mitterrand to master them in this year's balloting and if he can continue that mastery in the next set of legislative elections to carry out a widely reported personal vow made in 1972 ultimately to eliminate the Communists as a significant force in France.

That kind of tenacity has been a hallmark of the career of Mitterrand, who was born on Oct. 26, 1916, son of a stationmaster in Jarnac in the Charente region of southwestern France. A brilliant scholar who earned law and political science degrees in Paris, Mitterrand was wounded and captured by the Germans early in World War II. He escaped and returned to France where he helped set up a resistence network that brought him into contact with De Gaulle, who characteristically took an immediate dislike to the younger resistance fighter. De Gaulle appointed him to a high-level post after the liberation in 1945 but they soon parted ways.

Two years later, at age 30, Mitterrand became the youngest full minister of the Fourth Republic in the Socialist Cabinet of Paul Ramadier. Over the next decade, he bounced from ministry to ministry as governments rose and fell, until De Gaulle returned to power in 1958 and began the center-right rule that Mitterrand broke yesterday.