The 28-year-old, granddaughter of the far-right party’s founder Jean-Marie Le Pen and niece of party leader Marine Le Pen, temporarily withdrew from politics after her aunt’s presidential election defeat last May.
But she has long been viewed in France as a possible future leader of the National Front.
Ms Marechal-Le Pen was a keynote speaker at the Conservative Political Action Conference in Maryland, US on Thursday – an event Mr Trump and Vice-President Mike Pence also addressed.
During a fiery speech, she said Brexit and the Trump campaign provided powerful lessons for her own country and attacked the EU for stifling French freedom.
And, although her appearance was controversial because of similarities between the US alt-right movement and the National Front’s views, the crowd joined in with boos every time she mentioned the EU.
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She told the conservative audience: “I’m not offended when I hear President Donald Trump say ‘America first’.
“In fact, I want America first for the American people, I want Britain first for the British people and I want France first for the French people.
“I came here today to tell you there is a youth ready for this fight in Europe today.”
She added: “France is no longer free today.
“After 1500 years of existence, we now must fight for our independence.”
Her speech has been assessed in some quarters as the first steps in a leadership challenge to her aunt.
Ms Marechal-Le Pen, who is more socially conservative and economically liberal than her aunt, did not say if or when she was planning to return to an active role in French politics.
Instead, she mentioned her plans to open a management and political science school, which she said should train tomorrow’s leaders.
But the reception for the former politician, who has been praised by right-wing US media outlets like Breitbart, was in contrast to her aunt’s visit to Trump Tower in New York in January last year, during the French presidential campaign.
Marine Le Pen, who has long aimed to burnish her credentials with foreign appearances, did not meet either Mr Trump or anyone from his team during that visit and was photographed having coffee in a public area of the building.
While Ms Le Pen has over the past months softened her anti-EU, anti-euro comments, her niece lambasted the bloc, which she said was “slowly killing” ancient nations through immigration and political correctness.
She told the activists: “Let us build on what you have achieved here so that on both sides of the Atlantic, conservatism may prevail.”
Marine Le Pen is hoping that a National Front congress in mid-March will help her reassert her authority.