The 25-year-old went to Two Brewers gay bar in Clapham, south London, after the premiere of the Wizard of Oz show.
Church, who split from partner Gavin Henson last year, had previously judged the BBC1 reality show Over the Rainbow which chose the new Dorothy, Danielle Hope.
She is said to have sang Alannah Myles' hit Black Velvet while she danced around the stage.
According to British newspaper The Sun, an insider said: 'Charlotte was having a great night. She was letting her hair down and having a few drinks.
'There was karaoke going on and Charlotte got on stage barefoot and put everyone to shame by singing Black Velvet.'
The source added: 'She seemed to really enjoy it and got massive applause from everyone. After that she was mingling with everyone and when she popped outside she was sharing cigarettes with people.
'Everyone loved it and she belted out a really good rendition of Black Velvet.'