Many sources said then and now that she left the Hardy Boys/Nancy Drew Mysteries when the producers decided to have Nancy join the Hardy Boys for their episodes, instead of having her own segment like before. I believe that, because it was only after she left that the the character joined the Hardy Boys.

According to wiki she was kicked out by producers when she appeared in Playboy.

Same here.When I hear the name Nancy Drew,I automatically think of Pamela Sue Martin and how she used to make me melt when I,d see her on tv.I was just a kid then but just realized how much older she is than me,that surprised me,I thought we were near the same age,,and,I see she lives fairly close to me too,that is if she still lives in Idaho.The only reason I ever watched Dynasty was to see her,but never watched that show much,I remember her as Nancy Drew and of course her role in The Poseidon Adventure.I hope she,s doing well.
Illigitimi non Carborundum

If you watch the first season of HB/ND the Nancy Drew episodes were stand-alone. Second season ND was most frequently a HB sidekick. Others have said this was the reason she left. As for Dynasty she left because of a lifesyle altering(but not threatening) medical condition which I hope she has brought under control.

Right, she did that on Dynasty too. I forgot about that.
What man again?

PSM had a habit of leaving roles mid way!
Its that man again!!

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I live on the East Coast.
Sundays my Dad watched 60 Minutes, but we had more than one TV in the house so I'd watch HB/NDM on one of the other sets. I wasn't even a preteen yet. We didn't have VCRs back then.

I like the Nancy Drew and Hardy Boys DVDs. My parents used to watch it! I like Pameb68la as Nancy Drew. I am watching tthe first show with the new nancy. Ick.. I like pamela better!!

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Hey, I'm 40, too! We were young enough to appreciate good quality programs on broadcast television. I even remember when it was onSunday nights at 7PM on ABC.
I went to Pamela Sue's profile, she still looks good today!

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Season 2 of THE HARDY BOYS/NANCY DREW MYSTERIES just came out on DVD and features the episodes where she joined forced with the Hardys and, sadly, the episodes where Janet Louise Johnson assumed the role of Nancy Drew. I had a huge crush on her when I was a kid and I was quite heartbroken when she left the show. Pamela Sue Martin will always be my Nancy Drew.

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She wanted to change her image. She left the show and then posed nude for Playboy in 1978.