From IMDB:

The character of Mandy (played by Moira Kelly) was abandoned after the first season. Aaron Sorkin and Kelly both agreed that the character had run its course. However, Sorkin never explained on the show what happened to Mandy - the character just disappeared.

It is rather sad that the sudden disappearance of Mandy Hampton is never explained. After all, she has a history with Josh and he never seems to even remember her anymore. She is not one of my favorite characters, but I like her all the same.

Owing to this lack of any explanation, I have thought up some that might be preposterous, but better than nothing at all. And please bear in mind that all these remarks were made with tongue firmly planted in cheek.

1) The obvious one, especially when you read "conspiracy theory" is that Mandy was abducted by aliens because she was the most intelligent of the people in the White House, yes, even more intelligent than Bartlet or Leo or Sam. To prevent people from noticing, the aliens erased their memories about Mandy, and also took everything that might remind them of her (personal belongings, gifts like the photo she gave Josh, etc).

2) The whole first season happened in Josh's dream while he was lying between life and death on the operation table after being shot in Rosslyn. It's not that nothing of those things happened, it's just that Josh somehow mixed Mandy in there because deep inside he was still in love with her and wanted to see her working at the White House again. This is not unseen on television, as fans of St. Elsewhere and Dallas would concur.

3) The whole series except the first season happened in Mandy's imagination after being fired because of the memo she wrote while working for Lloyd Russell.

4) Mandy actually still works at the White House but she has different working hours than everyone else, that is, she only comes to work when nobody's around except the janitors and security and cooks. Her office is one floor downstairs from Ainsley Hayes's. The others honestly just forget about her because, as they say, "out of sight, out of mind".

Feel free to add your own :D

I just think it would have made far more sense if Mandy was killed in the end of season one, then at least there would have been an explanation.

Yes I agree with you, it would've been easy if they had just said Mandy got killed by a bullet. But then how do you explain her absence in the flashback scenes? Bartlet's senior staffs are people from his campaign, including Mandy I assume. How come she didn't appear in any of those flashback scenes?

I never really liked her as a character. Always thought she was 'swarmy and smug' there was just something that didn't work. Was glad when she left at the end of season one.

Yeah I didn't like her character eiter... I do like the alien abduction theroy. They should bring her back... then kill her off... and as far as the flashbacks... simple, she was NEVER around for the important stuff. HA HA.

I noticed this straight away when I watched the beginning of season 2 because I REALLY disliked MAndy, yeah i agree she was annoying, smarmy, insensitive (and not in the vunerable Josh way) and v. smug. However my theory was that she was shot in the shooting and coz nobody cared about her they all packed up after they found Josh and the President and forgot about Mandy.

I'm not that the West Wing staffers would be that insensitive, but I would agree with the general consensus that Mandy's character definitely didn't work.