Sarah Thomas, who is from Colorado, completed the feat Tuesday morning after more than 54 hours of swimming.
The 37-year-old completed treatment for breast cancer last year.
She was cheered by a small crowd as she arrived in Dover. She was given champagne and chocolate to celebrate.
Thomas says she was "a little sick" after so much time in the water and thanked her husband and supporters for urging her to keep going.
She has dedicated the swim to "all the survivors out there."
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