Not a household name, but had memorable guest roles on TV for decades; Cheers, Wings, Seinfeld, Northern Exposure. If you had a role that needed the perfect balance of adorable and crazy, she was the one you called.
She was hilarious in The Powers That Be. RIP.
I first saw her in Fresno. She’ll always be Tiffany to me.
Just saw her recently on Private Practice, which immediately reminded me of her character, Eve, on Northern Exposure. I had thought she had also been on Wings, and I just confirmed that my memory didn’t fail me - she played Sandy Cooper, described here as “Alternating between obsessively unhinged and calmly composed”.
She did do that well. RIP.
I had to look her up, because I didn’t recognize her name, but I sure did recognize her face! She often played a thorn in the side of the main character for an episode.
She was an “It’s THAT guy!” One of those people who was never a star, but still made a decent living as an actor, and didn’t need a second job.
RIP
Like you, I didn’t recognize her by name. but after doing an image search, I recognized her immediately. I could actually recall her voice!
Throughout movie and TV history, there have been lots of people like her, character actors who were constantly in demand. I can imagine a casting director saying “Get me Valerie Mahaffey. Or if she’s not available, get me a Valerie Mahaffey!”
A lot of people like her had careers lasting decades longer than some star-of-the-week who fizzled out as quickly as they emerged. Unfortunately, this happens to actresses more than actors.
For example, everyone knows Tom Cruise, but when was the last time you saw Rebecca De Mornay? (They starred together in “Risky Business.” She was going to be the next big thing, until she wasn’t.)
She has a recurring role on NCIS and is probably going to feature prominently in the coming season.
But yeah, she picked some lame movies to appear in.
Valerie Mahaffy was Sheldon & Georgie’s English teacher on Young Sheldon, but that’s the only regular role I can think of her having.
I was also thinking, because of the “shows that hold up” thread, that before she was cast as Lacey, Tyne Daly was an “It’s THAT guy!”
She was awesome as ‘Eve’ on Northern Exposure. I still remember when Maurice’s expensive vintage bottle of whiskey smashed. She glued the bottle back together and concocted ‘new’ vintage whiskey (adding a bit of this, a bit of that), filled it, capped it, and no one was the wiser.
She had a recurring role on Wings as a woman who obsessed with Joe Hackett since high school. Some of the best episodes among many other notables of this long running series. Quirky characters were her thing.
She also played one of George’s girlfriends on Seinfeld, the pretentious accountant with chopsticks in her hair who was doing Jerry’s taxes until George broke up with her.
I too had to look her up. Here’s her wiki picture, and that helped me.
Rest in Peace.
I remember Fresno. It was a spoof of all the Prime-Time soap operas that were on in the '80s, like Dallas and Dynasty. Carol Burnett played the “Matriarch” (her word, not mine) of the family. My girlfriend at the time tried to turn off the TV because I was salivating over Teri Garr in a Teddy. 
I remember Valerie distinctly because she at one point faced a lineup of men in clown outfits and said “Okay, which one of you bozos is my father?” 
ISTR one of the producers of Frasier said that while Maris was never intended to appear on screen, and they drew her flaws to ridiculous levels, if anyone could have played her, it would have been Valerie Mahaffey.
Mahaffey did appear in a single episode of the show, as Frasier’s receptionist.
First thing i thought of was her in Seinfeld and “papier mâché” !
Yes; I don’t know if that show is ever shown since I don’t think it hit the required 100 episodes for syndication. But it was epic.
As mentioned by others, Mahaffey was reliably good in guest roles. She always got the laughs that could be found in whatever her lines happened to be.
I remember her riding a horse and each hand holding a rein.
She was almost 72. I thought she was younger, but I’m probably losing track of time.
One of the first roles I remember seeing her in was a Newhart episode in which she played a doughnut shop clerk whom Larry had a crush on but couldn’t tell her, so he had Bob try to act as a messenger.
I didn’t recognize the name either, but I instantly knew several roles when I saw her picture. RIP.
I saw the first episode long ago, and thought it was hilarious. Didn’t know she played the daughter. As the OP put it, adorable and crazy.