The 32-year-old British actress obtained her citizenship alongside Camila Alves at a VIP legal oath ceremony in Los Angeles on Tuesday, reported Us magazine.
It comes five years after the "Into The Woods" actress married her Massachusetts-born actor husband John Krasinski.
Blunt - who was born in London - was spotted leaving the courtroom holding Krasinski's hand, accompanied McConaughey and Alves and their children, Levi, seven, Vida, five, and Livingston, two.
Blunt or her husband are yet to comment on the news, but McConaughey, 45, was so pleased his model wife had finally become an American he took to Twitter to tell the world.
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