Meryl, whose vocal talents were revealed during her turn on Mamma Mia!, carried a toy dog as she took on the role of Dorothy.
Sir Elton was the Cowardly Lion while Sting donned a metal hat to play the Tin Man.
The fun – which took place at New York's Carnegie Hall – was all in aid of the Rainforest Fund.
'Songs from the silver screen' was the theme of the gig, which also featured performances from Jennifer Hudson and Bruno Mars and was hosted by Sting's wife Trudy Styler.
Dreamgirls star Jennifer reprised her number from the film And I'm Telling you I'm not Going, while Bruno teamed up with Elton to belt out a rousing rendition of Unchained Melody from Ghost.
Elton took to the stage again to sing Diamonds are a Girl's Best Friend, after noting: "It has become a custom at these shows for me to humiliate myself."