Merkel had come under fire from critics at home for interfering in French domestic political affairs and Sarkozy has been accused of aligning France's interests too closely with those of its powerful neighbour.
France goes to the polls in less than 80 days, but both leaders played down the issue at a Paris news conference held after a joint Franco-German cabinet meeting and ahead of an unprecedented joint television appearance.
"I support Nicolas Sarkozy on all levels because we belong to political parties that are friends," a cheerful-looking Merkel said. "It is normal that we support parties that are friends."
Merkel recalled that Sarkozy had supported her in 2009 during her own election campaign, and said the French leader's Socialist opponent Francois Hollande had recently come to Germany to address the opposition SPD.
Sarkozy, also smiling, thanked Merkel for her "friendship and trust" and -- without citing him by name -- slammed Hollande's plan to renegotiate a eurozone fiscal compact seen by Berlin and Paris as a solution to the debt crisis.
France and Germany were once seen as the twin motor of the European Union, but Paris is now clearly the junior partner, its economy lagging behind by any measure, and Sarkozy has turned to his fellow right-winger for ideas.
Sarkozy trails Hollande in the opinion polls and Merkel fears that any new left-wing French administration will diverge from her austerity plans.
Specifically, Hollande has said he will seek to renegotiate the hard-won eurozone "fiscal compact" that Merkel sees as essential to binding the single currency bloc together behind her deficit-cutting agenda.
Sarkozy, still smarting from France's loss of its AAA debt rating, now cites Germany's success almost daily to justify his own policies, drawing inspiration from both Merkel and her centre-left predecessor Gerhard Schroeder.
In Berlin, Merkel's spokesman said her active support for Sarkozy's re-election campaign was a "personal" engagement as leader of Germany's centre-right party and not as the nation's chancellor.
Insisting that Germany is "of course not a party in France's election campaign", Georg Streiter said: "It's a question here of personal involvement by the CDU leader Angela Merkel."
Merkel, who heads the conservative Christian Democratic Union, took several ministers to Paris Monday for the joint Franco-German cabinet session and was to appear with Sarkozy on French and German public television later in the day.
"It's not a question of an election campaign event but a German-French council of ministers and it's about important political topics," Streiter told a regular news briefing.
According to CDU general secretary Hermann Groehe, Merkel plans to support Sarkozy at campaign rallies ahead of France's two-round presidential election, with a first round on April 22 and a run-off on May 6.
Sarkozy's opponents have accused him of toeing Germany's line and even some in his own camp are worried about his close adherence to Berlin's views.
"We must be careful not to appear to be following Germany's lead," said Lionnel Luca, a UMP lawmaker, warning against the "whiff of Germanophobia exuded by the Socialist Party for political reasons."
Hollande has taken a strong lead ahead of the French presidential vote, with polls showing him taking 28 to 30 percent of votes in the first round, against 23 to 24 percent for Sarkozy.