Presenting the costume award along with Brian Tyree Henry, McCarthy, a nominee for best actress, appeared in an elaborate royal get-up, channeling Olivia Colman's Queen Anne in "The Favourite", including a cape embellished with stuffed rabbits - a reference to the queen's beloved pets.
"I used to work as a cleaner and I spent quite a lot of my time imagining this".
She is the only British victor of one of the main acting prizes, despite representatives in each of the categories with Christian Bale nominated for best actor in Vice, Richard E Grant up for best supporting actor in Can You Ever Forgive Me? and Rachel Weisz nominated for best supporting actress in The Favourite. "And when you think it's impossible, just do not forget to say this piece of advice I got from a very wise woman, 'I did my best, and my best is good enough'".
"Any little girl who is practising their speech on the telly, you never know".
Olivia Colman won Best Actress during Sunday's Academy Awards for her performance as Queen Ann in "The Favourite", and was visibly stunned as she took to the stage to accept the award.
Comedian Rebecca Humphries echoed those sentiments, criticising a recent newspaper description of Colman as an "actor who early in her career struggled for parts and was best known for comedy".
Olivia Colman is the victor of the best actress Academy Award for "The Favourite".
"My husband Ed, my best friend I love you so much, he's going to cry".
Fellow Oscar victor Julia Roberts took to Instagram to share her appreciation for Colman, writing: "I (love) Olivia".
Haig wrote: "Last November, Olivia Colman was asked - amid her busy schedule - to do the audiobook for the Truth Pixie".
An emotional Colman thanked her three children - ages 13, 11 and 3 - who she said were watching from her agent's house in Los Angeles. "This has been a long time coming", she said, laughing unabashedly.