After François Hollande's victory in the presidential election last month, the Socialists – who already dominated the Senate – took control of the National Assembly by winning 314 out of the house's 577 seats.
The result means they will not need to rely on the Greens or the far left to pass laws.
The far-right National Front was set to return to parliament for the first time since 1998 after winning at least two seats in the south of the country, although party leader Marine Le Pen lost her own bid for a seat.
Hollande, who defeated right-winger Nicolas Sarkozy in a May presidential election, had urged voters to give him the MPs he needs to steer France through the eurozone crisis, rising unemployment and a faltering economy.
"The task before us is immense," Prime Minister Jean-Marc Ayrault said late
Sunday as results from the second round of the legislative vote still trickled in. "Nothing will be easy."
Beyond Hollande's election promises of job creation and tax hikes, the government will have to pass unpopular measures to bring the deficit below three percent of GDP.
Hollande was due to hold G20 talks in Mexico Monday, flush with electoral success and brandishing a further mandate to push for growth strategies – rather than austerity measures – to battle the eurozone's debt crisis.
He has also floated a proposal for a 120 billion euro ($150 billion) "growth pact" to be discussed at a series of high-level meetings ahead of a European Union summit on June 28-29 in Brussels.
The man Hollande beat to the country's top job, Nicolas Sarkozy, saw his centre-right UMP lose more than 100 seats to keep only 194, while the centrist party of once high-flying François Bayrou won only two.
Le Pen, who has said her success in the first-round parliamentary vote made her party France's "third political force", demanded a recount after she was narrowly defeated by a Socialist in a northern former mining constituency.
But the telegenic Le Pen nevertheless rejoiced in the overall success of her party, whose image she has fought to soften from the days of her father Jean-Marie's provocative outbursts.
"This is an enormous success," Marine Le Pen said in Henin-Beaumont.
Le Pen's niece, 22-year-old Marion Maréchal-Le Pen, won her seat in the southern Vaucluse region, becoming the youngest MP in post-war France, and the FN won another seat in the nearby Gard constituency.
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The media spotlight was also focused on Ségolène Royal, Hollande's former partner and mother of their four children, who conceded defeat in her battle after a dissident Socialist candidate refused to stand down.
Royal, whose campaign was shaken when Hollande's current partner Valerie Trierweiler tweeted her support for dissident Olivier Falorni, slammed what she called a "political betrayal".
The Socialists and allies won 50.34 percent of votes overall, interior ministry figures said, almost as high as the record 54 percent won shortly after Francois Mitterrand became France's last Socialist president in 1981.
As result estimates came in, UMP leader Jean-Francois Cope said he "took note of the left's victory" and said his party would constitute a "responsible and vigilant opposition."
With the French voting for the fourth time in eight weeks after electing their first Socialist president in 17 years, turnout was a record low for a second-round parliamentary vote at 56 percent.