The couple, who have been dating since last summer, were married by an Elvis impersonator at the celeb-friendly Graceland Wedding Chapel, reports The Sun – who have obtained their wedding certificate.
The news comes after showbiz gossip website TMZ reported that the pair had obtained a marriage licence, which would allow them a year to tie the knot.
Since the story broke, Allen has since shared a number of images of the low-key big day on social media.
Wearing a white bardot-style mini dress and a veil, the star looked loved-up as she shared a snap laughing with Harbour, being serenaded by an Elvis impersonator and chowing down on a burger.
While the singer, 35, and Stranger Things star, 45, have been pretty private about their relationship, Allen appeared to confirm engagement rumours earlier this year.
The star has been sporting a seriously impressive diamond ring on her wedding finger and Harbour recently joined Allen and her children, Marnie Rose and Ethel, for a family holiday in Croatia.
Their relationship appears to have begun in 2019 when they were spotted enjoying a number of dates in London.
They then took things Stateside when Harbour made his Saturday Night Live hosting debut, with Allen flying to New York to join him and celebrate.
Their red carpet debut came in January when they attended the Screen Actors Guild Awards together.