What was her career/reputation like pre-1971?

It sort of seems (to the Leachman uninformed like me) that she suddenly became a respected comedian and Oscar winning dramatic actress all of a sudden.

She worked a lot pre-1971. It looks like the most notable role for her up to then was playing Ruth Martin in 28 episodes of Lassie.

https://www.imdb.com/name/nm0001458/

Miss Leachman has worked steadily if not always prominently as a performer ever since she won Miss Chicago in 1946!

One thing about her actual origins that seem a bit unconventional is that in her bio she revealed that she found out in her teens that her namesake mother had been married to a man other than her legal father that Miss Leachman had never previously heard of when she was conceived but that that Mr. Leachman WAS her actual father.