Mayo made the announcement in a room full of supporters in Monroe.
Mayo says his time leading the city of Monroe makes him the right candidate for voters in the fall.
"We know how to do it. As a Mayo, we know what those issues and challenges are. We believe we can lend that experience throughout the 5th Congressional District."
Mayo will make a similar announcement this afternoon in Alexandria, the second biggest city in the 5th Congressional District.
So far Mayo is the only Democrat to announce his candidacy for the race.
On Tuesday he picked up the endorsement of Democratic State Representative Marcus Hunter.
Both ran during last year's special election, with Mayo finishing third and Hunter finishing seventh.
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