Spencer told host Jimmy Fallon on Monday she met McCarthy at the Groundlings Theatre & School in Los Angeles where McCarthy would perform. The pair then went on to enjoy a 20-year friendship together.
"Octavia would come to see our shows and she had, not surprisingly, her laugh is so infectious. It's so great that our shows would go better," McCarthy said.
"Nobody would want to go do a show unless she was in the audience because everybody laughed more and then that was it. We've been stuck together ever since, delightfully," McCarthy continued.
McCarthy also teased an upcoming Netflix film she stars in with Spencer, titled Thunder Force. The project, from McCarthy's husband Ben Falcone, features the duo as friends who become superheroes.
"I just want to do films with Ben and Melissa and laugh everyday," Spencer said of her experience working on the film.
Spencer and McCarthy were challenged by Fallon to a best friends quiz to see which actress knew each other the best.
Spencer and McCarthy answered the questions incorrectly, however, and pretended not to know each other well.