The Glaswegian potter endured a miserable match and failed to reproduce the form which saw him send Stephen Hendry into retirement after a 13-2 quarter-final triumph.
Rarely did the run of the balls favour Maguire, who lost to John Higgins in his only previous Crucible semi-final five years ago, as 32-year-old Carter turned a 14-10 overnight lead into a 17-12 triumph
When he lost a dramatic opening frame in yesterday's first session the Scot's hopes were effectively over. Carter, with a 21-point lead, was put in a clever snooker behind the black, which was by the side rail near the yellow spot.
Carter tried six times to strike the final red and failed, and on the final occasion referee Leo Scullion elected not to call a miss. Carter had been a whisker away from hitting the ball, so Scullion's judgment meant his fellow Glaswegian, by that stage three points ahead, had to play from where the white finished.
The double was on but the red wriggled out of the jaws. Carter produced an outstanding pot to the far left corner and soon took the frame.
Maguire's head dropped and he soon lost the next frame, Carter having an outrageous fluke red in the process. The Scot rallied with back-to-back frames, but once they returned from a mid-session interval, Carter leapt at an opportunity which presented itself after Maguire potted a long red but followed it into the pocket with the cue ball.
He rifled in a break of 70 and Maguire offered his hand. For Carter, today's final appearance marks an amazing turnaround after suffering so badly with Crohn's disease, the bowel condition, that he considered walking away from the game.
Carter admitted reaching the final had surpassed his expectations. He said: "I'm as surprised as you are. I'm delighted to be through. The job's not done yet, though."
Maguire admitted he had underestimated Carter, saying: "I think in the first session I didn't give him enough respect."
Carter will today face title favourite Ronnie O'Sullivan who made short work of Matthew Stevens in the evening session, winning the two frames he required for a 17-10 victory. O'Sullivan will be going for his fourth title.