The foursome, who were put together from the solo entrants at the audition stage, have become the first group ever to win the ITV1 talent show.
They were jubilant on hearing the news, only able to screech and say "oh my god!" for a couple of minutes.
Then Jesy managed to just about get a sentence out, saying: "This is never gonna sink in, we cannot thank the public enough."
The girls had emerged as the favourites over the last week, also seeing off the previous favourite Amelia Lily in the Saturday night show.
There were no hard feelings from Marcus though, and he said: "These girls really deserved it."
The "Little Muffins", as mentor Tulisa rather embarrassingly referred to them, performed a memorable version of Silent Night - prompting some wild praise from judge Louis Walsh.
He said: "I'm going to predict this time next year this is going to be the biggest girl band in the UK by a mile."
You want our predictions? OK then: Kelly won't be back next year (y'all). Tulisa will guest on several Little Mix tracks and then join them when her solo career falters, Marcus will be in Chicago on the West End stage inside 18 months.
Oh, and Tulisa's sideways arm gesture will still look rubbish next year.