Manuel Valls, 54, stunned his Socialist colleagues by saying that he wanted to stand in June's parliamentary elections as a candidate on Mr Macron's side.
The centrist's emphatic victory, which also smashed the dominance of France's mainstream parties, will bring huge relief to European allies who had feared another populist upheaval to follow Britain's vote to quit the EU and Donald Trump's election as US president. Macron has promised that half of those candidates will be new to elected politics, as he was before his victory Sunday.
Split 50-50 between men and women, they'll have Macron's example for inspiration: Contesting his first election, he handily beat Le Pen with 66 percent of Sunday's vote and tore up France's political map.
Although many across Europe were relieved that the anti-immigrant euroskeptic National Front (FN) leader Le Pen was defeated, many in France felt deflated after they felt they had chosen "the lesser of two evils", as one French voter put it to DW.
Emmanuel Macron started his first full day as president-elect on Monday at the side of current French President Francois Hollande.
Marine Le Pen has congratulated Macron on his victory.
With legislative elections just five weeks away, the start-up political movement the 39-year-old former investment banker launched one year ago on his meteoric ride to become France's youngest president lost no time Monday in girding for the crucial mid-June election battle.
Hollande will delegate his powers to Macron on Sunday, May 14. But a high number of blank or spoiled votes and unusually low turnout are signs of an electorate dissatisfied with its choices. Le Pen called for France to leave the European Union and drop the euro currency in favor of the franc.
Sylvie Goulard, a French deputy to the European Parliament, said Mr Macron would make Berlin his first official visit, with perhaps a stop to see French troops stationed overseas as well.
But National Front vice-president Florian Philippot said Ms Le Pen remains the "uncontested" leader of the party even after her heavy defeat. In interviews on Monday her campaign director David Rachline said the party founded by her father would get a new name as bait to pull in a broader spectrum of supporters in France.
Trump, who had hinted at support for Le Pen during the campaign, tweeted congratulations to Macron on a "big win" and said he looked forward to working with him.