Mary Ellen (Judy Norton Taylor) is the oldest of Liv and John's daughters, born in April 1920, aged 13 in season one. Throughout the first few seasons, she is a typically whiny, sometimes rebellious teenager, somewhat of a tomboy who enjoys playing baseball, but could also be vain, engaging in a rivalry with rich-girl Martha-Rose Coverdale for the affections of the awkward G.W. Haines (David Doremus). Mary Ellen matures into a wiser young woman and her childish fantasy of becoming a movie star gives way for a more reasonable and realistic ambition to go into medicine after reading up on it and developing an interest. She then works to gain an education as a medical worker, and becomes a nurse. However, when she ends up taking care of the people out in the country by herself, she concludes they need more medical expertise than she can offer them and continues studying medicine until she succeeds in becoming a fully-fledged doctor. Even though some people frown upon female doctors and she receives mixed support from her family, she refuses to let this stop her. Mary Ellen has a special relationship with each of her six siblings, but she is especially close to her younger sister Erin. Mary Ellen and Erin fought a lot when they were younger girls, particularly in seasons 1 and 2. But in the middle seasons, Mary Ellen and Erin matured and became friends. In season 5 after Mary Ellen married Curt, her relationship with her sister deepened even further and by the end of the show, they truly did become each other's best friend.

Is it true that does mary ellen become a doctor on the waltons?
yes