The Irish teenager forgot some of the words to Nineties hit Mmm Bop on Saturday night's show, later blaming the mistake on feeling like she was "going to throw up".
Her second song, Red Hot Chili Peppers' Under the Bridge, was marginally more successful but not enough to save her from the sing-off with Mancunian Misha B.
Janet, 17, hoped singing Chasing Cars by Snow Patrol would be enough to win over the judges, but they preferred her rival's version of Out Here On My Own from the musical Fame.
Tearful mentor Kelly Rowland was unable to choose between her two acts and abstained from voting, having admitted "feeling sick" when both girls ended up in the bottom two.
Louis Walsh had already decided to save Misha B, however, saying: "I'm going to pick the person who's going to give the most to this competition."
He was followed by Tulisa Contostavlos, who voted for Misha after saying she wanted to save the person "who's going to sell records, who's going to sell out arenas".
Kelly's decision to abstain meant Janet was automatically sent home as the majority had already chosen her to go - but Take That singer Gary Barlow later admitted he would have also saved Misha B.
Asked how she felt at the news, Janet said: "I don't know. Obviously I'm very confused.
"I can't wait to walk out of the show and get my music out."