This Variety article sums it up pretty well.
She was a multiple Emmy winner and an Oscar winner for The Last Picture Show
. One thing the Variety article does not mention is her turn as Billy Mumy's terrified mother in the Twilight Zone
episode, It's a Good Life
. She and Mumy reprised their roles in a newer (not the most recent) incarnation of the Twilight Zone
From the article, "The daffy, self-absorbed Phyllis, a character she claimed was close to her own persona, brought the actress two Emmys as a featured actress in a series during the mid-’70s and made Leachman a household name."