The novice political party and its Mouvement Démocrate (MoDem) allies won a combined 32.3% of the vote, while the established Les Républicains trailed with 15.8% of the vote.
The scale of En Marche's win was marred only by poor turnout -- only 48.7% of eligible voters went to the polls.
Such a margin of victory in the 577-seat house would give Macron the majority he so badly craves to further his political revolution. The 39-year-old's La Republique En Marche (LREM) party is hoping to make huge gains and inflict a further blow on the country's traditional ruling parties.
It would be a remarkable achievement for Macron, who won the French presidency last month without the support of a traditional mainstream party.
Instead, his En Marche! movement helped carry him to a convincing election victory
over far-right candidate Marine Le Pen.
Le Pen's right-wing Front National party garnered 13.2% of the vote Sunday, but looked to win only a smattering of seats.
The Socialists, the party of outgoing President Francois Hollande, won just 7.4%.
Macron's party contested 526 constituencies out of a possible 577. His party put forward 266 women candidates, while 219 come from outside politics. He has drawn candidates from a cross-section of society, including a former bullfighter, a Nobel Prize winner and an ex-fighter pilot.
The move appears to have paid off as Macron's party, which has grown out of his grass-roots movement, is projected to record a stunning victory.
"We are grateful for the trust you have placed in all the new faces of the Republic," Catherine Barbaroux, the party's president, told supporters after seeing the projected results.
How the elections work
The final results will be confirmed next Sunday after the second round of voting. A number of candidates face a run-off.
To win a seat in the first round of voting, candidates had to win more than half of the votes, which must account for at least a quarter of the registered voters.
If no candidate manages to achieve that target, then all candidates who won at least 12.5% of registered voters go to the second round, where the winner will advance to Parliament.
Both the Republican and Socialist parties, which have traditionally governed during the time of the Fifth Republic, struggled with turnout.
The National Front is expected to take between one and four seats, while Jean-Luc Mélenchon's far-left party
is expected to claim between 10-20 seats after accounting for 11% of the vote.
Florian Philippot, the deputy chairman of the National Front, said his party was "disappointed" with the result.
"We've maybe been disappointed by the score and we have paid the price, I think, for a low turnout," he told reporters.
Why is this important for Macron?
The success of those parties, however, is likely to pale into insignificance should La Republique En Marche secure the mandate Macron requires to govern successfully.
Macron, who won the presidency by being a pro-European centrist, is hoping to carry out far-reaching reforms to overhaul the country's political system and economy.
France is suffering from high unemployment, a stagnant economy and security worries. The government has also struggled to cope with immigration and integration.
But for him to be able to implement his reforms, he needs to be able to govern -- and that means having a majority in Parliament.