Carter moved within one frame of a spot in the final by claiming the opening two frames of the afternoon to lead 16-10 helped by a fluked red in the 26th frame only for Maguire to keep the contest alive by winning the next two frames aided by 56 and 53.
An impressive knock of 70 in the 29th frame from Carter was good enough to see the Essex potter over the finishing line.
"I didn't have a lot of expectations coming here," said Carter. "I stuck to my guns out there and I am pleased to be through."
Carter has ousted pre-tournament favourite Judd Trump and former UK winner Maguire to reach the final. He is likely to face Ronnie O'Sullivan - who leads Matthew Stevens 15-9 in the other semi-final - in a repeat of the 2008 World Championship final, but looks to have his work cut out against the three-times world champion.
O'Sullivan overcame Carter 18-8 in the final four years ago, and has yet to suffer defeat to his fellow Englishman in 11 meetings.
Ali Carter closed to within three frames of his second Crucible final appearance as he took a 14-10 lead over Stephen Maguire.
Carter had been seven frames clear at 14-7 in their semi-final and even had the possibility of wrapping up a final berth with a session to spare but Maguire showed fighting spirit to keep his own hopes alive by winning the final three frames.
Resuming at 10-6, Maguire was first in the balls in frame 17, knocking in a nice opening red before opening up a substantial early lead with a 45 break. Carter seemed to have the measure of a lengthy opening safety exchange but an outrageous fluked red into the middle pocket proved sufficient for Maguire to clinch the opener.
The Scot was yet again first among the balls in the next after a lengthy safety exchange, Carter’s ambitious plant scattering the reds everywhere and allowing Maguire to pull out a 44 point lead. But a slack safety let Carter in and a break of 53 and a tough missed final red along the side rail led to another tense safety exchange. Both players missed chances at yellows but Carter knocked it in at the second time of asking to restore his four frame advantage.
A bad safety shot from Maguire let Carter in to take full advantage in frame 19 and he punished his opponent with a superb 134, his fifth century of the tournament, to forge five frames clear.
As with the opening two frames, Maguire was in first looking like a man struggling for confidence and rhythm in the next but twice ran out of position to allow Carter to take the opening session 3-1 for a 13-7 advantage.
The interval did not seem to have any effect on Carter as a break of 73 took him further clear. But Maguire reduced the deficit with a gritty 72 break and a further 70 pulled him closer.
Carter built a lead in the final frame of the evening and looked to be heading to a 15-9 overnight lead before a tough missed red let Maguire in again and despite a small wobble, the Scot won his third on the trot to make things interesting when play resumes at 2.30pm on Saturday.
Ali Carter pinched the final frame of an exacting second session to extend his lead over Stephen Maguire to 10-6 in their World Championship semi-final at Sheffield's Crucible Theatre.
Maguire levelled the match at 5-5 with runs of 57 and 59 having trailed 5-3 overnight only for Carter to win the next four frames to move 9-5 clear. Maguire hit back with a break of 142, but narrowly lost the final frame of the session.
After Maguire's initial fightback, Carter made 118 in the 12th frame in re-establishing his two-frame lead at the mid-session interval before holing a tough black to steal the 13th frame after Maguire had picked up the two snookers he needed to win the frame.
With Carter conceding four more points in fouls, Maguire seemed destined to close to 7-6 behind only to see the white hit the knuckle of a middle jaw and rebound towards baulk after he had slotted the pink to the green pocket in travelling towards the black.
Maguire missed a long attempt at the black, but Carter was not so wasteful moments later. A composed 65 in the 14th frame was enough to ensure Carter would lead by at least two frames heading for the evening session.
2004 UK champion Maguire halted the bleeding with a run of 142 in the 15th frame - the second highest break of the tournament behind fellow Scot Stephen Hendry's 147 maximum in the first round - but a loose shot on the final yellow enabled Carter to clear to the pink to snatch a 16th frame that could have gone either way.
The pair resume for their third session from 7pm with Carter chasing a spot in the final for a second time after losing to Ronnie O'Sullivan in the 2008 tournament. .
He could face fellow semi-finalist O'Sullivan in this year's final, but it would be foolhardy to dismiss Maguire's prospects in the best-of-33-frame contest with both men playing in fits and starts for most of the opening two sessions.
Ali Carter leads Stephen Maguire 5-3 after a scrappy opening session of their World Championship semi-final at Sheffield's Crucible Theatre.
With the Crucible moving from a two to one-table set-up for the semi-finals and final, both players struggled badly with the pace of the cloth in just under three hours of play.
Scotland's Maguire - who last appeared at this stage in losing to John Higgins in 2007 - was not complaining as he made slightly fewer errors than Carter in winning the opening two frames of the contest.
Carter restored parity by picking up the next two frames with his opponent failing to pot a ball as a 46 in the third frame and 91 in the fourth frame enabled him to level at 2-2.
2008 World Championship finalist Carter - who ousted pre-tournament favourite Judd Trump in the second round - won his third straight frame despite Maguire getting a snooker he needed by rolling the white behind the black with just the pink and black left up.
Carter missed the pink before seeing a white fly into a top bag attempting a safety shot to leave Maguire with ball in hand and needing pink and black for the frame.
The 2004 UK champion holed the pink, but did not get desired position on a black that he jawed as Carter slotted black to escape from the frame with a 3-2 lead.
Maguire had only made a top break of 21, but came up with a run of 82 to level at 3-3 only for Carter to win the final two frames of a largely forgettable session - aided by a 65 in the seventh frame - to carry a lead into the second day of the match which concludes on Saturday afternoon.
They resume at 10am on Friday morning with the first man to 17 frames reaching the final. The winner will face Ronnie O'Sullivan or Matthew Stevens in the final with the second semi-final starting out at 7pm on Thursday.