The main thing that needs work in this piece is that the paragraphs are exceedingly long.  They need to be broken down into smaller units, it's just too much information in once place.

There are also several run-on sentences.

His face appeared on the front page of the Oakland Tribune and then the LA Times and then the NY Times four years ago when in his first magic show for a bunch of college friends at Berkley he made his girlfriend disappear.

This could be broken into three, possibly four separate sentences.  Or, it could be skimmed down into two separate sentences.  You don't want to overburden the reader with too much information in one sentence.  The structure needs to flow in a pace that's easy to digest, such as

"His face appeared on the front page of several newspapers years ago.  His first magic show was for a group of friend at Berkley.  He made his girlfriend disappear during the show."

Grammar is a factor, too.  Your use of quotations are wrong in a few places.  When not using direct dialogue, you place the period outside of the sending quotation mark; this is wrong.  It's a minor fix, though, and it's the only one I can really see.

The pace and background of the story is great.  I love how you talk about his life, how he got to where he is, and I especially enjoy the way you end it.  Absolutely great way to end the story.  Overall I enjoyed the story.

This was wonderful!  I loved every bit of it.

The only critiques I have are nitpicky minor stuff.

  He could hear piano from the stage. His opening act was just finishing (she could play piano with her toes – he didn’t hire her himself) and he was still in his dressing room, staring at the fake wood on the vanity in front of him. The mirror was covered in scratch marks and almost useless to him. He didn’t need it, anyway. He wasn’t one of those magicians who put makeup under their eyes to look mysterious.

Sentence one is ambiguous: It could mean he's either on the stage or not.  A minor thing, but potentially confusing right at the start before he's placed.  The stuff about the mirror is also useless.  If he doesn't need it, it's more than "almost useless to him".  It's entirely useless to him!  Either make him need it or change the following sentence a bit.  It does do a good job of setting the feel of the place he's playing.

The first paragraph could be split off into two starting at "One time", but it doesn't necessarily need to be.

Old Jack didn’t really wash away the memory as much as he framed it in an amber-tinted haze

I'm also not sure you can frame something in a haze.  "So much as cover it with an amber-tinted haze"?

He stood on that stage for thirty minutes, pulling hankie after colorful hankie out of the sleeve of his thrift store suit, the audience leaving one by one until a few gawkers were left. The hankies never stopped coming. He eventually took the jacket off, set it on fire and walked off of the stage.

This was great!

His face appeared on the front page of the Oakland Tribune and then the LA Times and then the NY Times four years ago when in his first magic show for a bunch of college friends at Berkley he made his girlfriend disappear.

This sentence is indeed a bit rambly, but I think you can cut the extraneous and make it manageable.

picked up by an agent who "knew how to make things happen”. Her words

Putting it in quotes should make it fairly obvious it's her words, not his.

She had agreed – foregoing important studying and maybe ruining a gradeher studies – because she loved him

This was a bit too maudlin.  I doubt one night of not studying is going to make you fail a class.

He never worked with people. But he had introduced a new trick to his routine a few months back.

This sets up for the ending, but if he'd done it "a few months back", wouldn't it have taken effect by now?  I think this would be a lot more effective if you changed this to something else.  The ending already packs quite a punch, but I think it could be a bigger one with a sentence change here at the end.

“Hello,” he said to the crowd of faces he could not see, “my name is Jacob Noon, and tonight I will make myself disappear.”

Absolutely love the ending.  I wasn't expecting it!

The brit in me disagrees with The Geek regarding quote marks.  It's only the USA that does things like that all the time! The rest of the english-speaking world doesn't.

Wonderful story!  Keep up the good work, and keep working on the good work, too.

 hi, jenn--

once again, you've charmed and surprised me with your writing, with this tale of magic and loss. you never fail to add a tinge of humor along with the pathos, and it makes for a wonderful read. i don't wanna hear you talk about not having great ideas all the time. *shakes finger*

 He could hear piano from the stage. His opening act was just finishing (she could play piano with her toes – he didn’t hire her himself) and he was still in his dressing room, staring at the fake wood on the vanity in front of him. The mirror was covered in scratch marks and almost useless to him. He didn’t need it, anyway. He wasn’t one of those magicians who put makeup under their eyes to look mysterious. He hardly brushed his hair. One time, he had wandered onto the stage after a four day conversation with his good buddy Jack Daniels, and the crowd loved him. Everyone wants to see a train wreck. That had been the show after the handkerchief incident. Old Jack didn’t really wash away the memory as much as he framed it in an amber-tinted haze. It was just an old gag with a hankie. Pull one out, woops, there’s another, and so on for about twenty seconds, then it’s old and you move on. It didn’t work that night. There was no moving on. He stood on that stage for thirty minutes, pulling hankie after colorful hankie out of the sleeve of his thrift store suit, the audience leaving one by one until a few gawkers were left. The hankies never stopped coming. He eventually took the jacket off, set it on fire and walked off of the stage.

i'm intrigued by the "the mirror was covered..." sentence. so mysterious! and not a little ominous. hm. the bolded spots regarding his hanky trick gone awry were moments at which i guffawed (yes, you made me guffaw, damn you). his unpredictable moments of true magic begin to peek out of the story. great character description, telling without spelling out what kind of person jack is. i also like jacob's disclaimer about the piano player--funny.

 He was Jacob Noon. His face appeared on the front page of the Oakland Tribune and then the LA Times and then the NY Times four years ago when in his first magic show for a bunch of college friends at Berkley he made his girlfriend disappear. The police staked out her workplace and dorm building for a month. They kept him under surveillance for two. They eventually let him go – very reluctantly. They thought he killed her somehow, and the magic show was just a ridiculous excuse. He wished he had killed her, sometimes, staring at the bottom of a bottle on some corner in Bumfuck, USA. Then maybe he would have been thrown in jail instead of into the limelight of professional stage magic, picked up by an agent who knew “how to make things happen”. Her words. That normally involved selling him as “that guy who made his girlfriend disappear”. That worked for two years. Then people stopped caring. They asked, “Who?” He got smaller and smaller gigs.

i love that you wait until the second paragraph to tell us jacob's name. this kind of off-beat rhythm you've mastered makes me wanna get out my notepad and study your methods. and what's the significance of jacob's last name? there's got to be something to it, right? i'm frustrated because i can't figure it out. jacob's story is tragic, even more so for his brief success caused by another unpredictable spasm of true magic. his fault that his girlfriend is gone, but no one would believe the truth, so he soldiers on to fame in the face of it. man, what a great depressing idea. and then his success fades, leaving him with close to nothing. except the guilt. that's still sitting on his chest. i LOVE this. parts of this story are very funny, but at its heart it's a story of debilitating grief and that just gets me. woof.

i was gonna mention the adverb action, but decided the second person pov fixes that, the casual relating of the story matches and it feels all right. (unfortunately, the system won't let me un-underline them.)

And every once in a while, something would happen. A dove would appear out of thin air over the audience. He would be able to guess cards without any tells or cheats, hours of correct answers, one after another. Spontaneous, unending handkerchiefs. One time he had tried that David Blaine levitating shit – for kicks, you see, drunken spur of the moment – and he had floated right out over the audience. That had booked him solid for three months. He never tried it again.

            He refused to work with people. He got mail every day from hopeful young women dying to become assistants, emphasizing their years of acting experience and their ability to think on their feet. He never bothered to reply. The answer stood at “No.” He would never work with people again. That night four years ago, he had begged Jessica to be part of the show. Her part would take ten minutes, he said. All she had to do was stand in a little box for a few minutes then “disappear” at the right time. She had agreed – foregoing important studying and maybe ruining a grade – because she loved him and wanted to see him do well at his first show. And he had made her disappear. He would never see her again. Her mother would never hear her voice over a long-distance phone line. Her little sister would never trounce her in Monopoly.

i understand that the consequences of his on/off magical abilities leave him unwilling to risk other people again, that when odd things happened during his shows he'd not attempt those tricks again...i totally get why, and it makes perfect sense and i even like that this mystery is never even close to solved...but that tiny voice in my head is screaming about it. it wants to know. even knowing it'd wreck the story, change the timbre of the whole thing, the plot and theme and fantabulous ending...it's still bugging me about it. no suggestion here, i'm just venting.

i'm not sure the underlined detail above is necessary or additive to the story. that his girlfriend was inconvenienced....ooh, wait. unless that was some subliminal hint about how his magic worked. talk about passive-aggressive. no? all right, it's a mildly interesting bit of background about his girlfriend, a tiny window into their relationship, but would it add to or alleviate his feelings of guilt over what happened? i dunno. maybe i'm overthinking this. but see, this is why every word is so important in these flash stories--i give each of them weight, assuming it's significant, trying to figure its place and meaning. if it's not, then you're just screwing with my head.

The piano on the stage stopped and there was a spattering of applause. He heard someone speaking over a microphone – he didn’t understand, but he thought he heard his name in there somewhere. He stood, throwing his threadbare suit jacket over his shoulders and gripping the doorframe, then wandering out to the small hallway that led to the stage, dragging along the wall. Someone put a microphone into his hand before he stumbled out into the blinding lights. The audience was very quiet, staring at him

okay, now we're back in real time and jacob is about to perform. poor drunken, wreck. great details here bring his state of mind to life, and state of living.

He never worked with people. But he had introduced a new trick to his routine a few months back.

            “Hello,” he said to the crowd of faces he could not see, “my name is Jacob Noon, and tonight I will make myself disappear.”

and this wamboozle of a twist. a sinister end for jacob to assuage his responsibility for destroying his girlfriend, her family's lives, his own. i'm thinking he introduced the act a few months ago, and hasn't been successful to this point, that something tells him he's going to pull it off tonight. and this last line is just perfect. the quintessential reveal, the mind-bending twist to the story--i'm satiated and grinning. i love those perfect moments.

you have a way of writing about people which gets under the surface--those layers i wish i could pull off with such finesse. i love the dissecting, and also that i can also choose to just read for surface entertainment and find that also. thanks for another great effort, jenn! you're building a formidable portfolio.