Cloris Leachman only did the first season. She was replaced by June Lockhart in season two.
She did part of the season that aired in 1957 - 1958. Ratings suffered and she was hard to get along with (perhaps because she didn’t want to do this role) and she was let go. June Lockhart replaced her until 1964.
Thanks. I did not know that. June Lockhart is iconic in that role. It was a good switch.