The 19-year-old defender saw Paul Scholes deflect home his 77th-minute 30-yarder to deny Sir Alex Ferguson's men a fourth successive league win.
• Edgar thrilled with leveller
James Milner had blasted the Magpies in front with a 33rd-minute piledriver, but a Scholes double either side of half-time gave the visitors the upper hand.
However, Glenn Roeder's injury-hit side summoned up their last remaining resources as Edgar marked his home debut with his first senior goal for the club to spark a gritty rearguard action to the final whistle.
United nevertheless extended their advantage over Chelsea to seven points ahead of the Londoners' trip to Aston Villa on Tuesday night, but it was the black and white shirts among a crowd of 52,302 who went home the happier after witnessing a display full of character and application.
The signs were ominous as the league leaders walked out at St James' Park determined to pile the pressure on Chelsea.
As Roeder attempted to put patches over patches after losing former United midfielder and prospective emergency central defender Nicky Butt to an ankle problem, opposite number Ferguson wheeled out his big guns again.
Gary Neville, Nemanja Vidic, Ryan Giggs, Scholes and Louis Saha were among the seven men he drafted into his starting line-up and there was a sense of trepidation among the home fans as the game kicked off.
However, while they had to defend for their lives with 19-year-olds Paul Huntington and Edgar and 20-year-old Steven Taylor joining midfielder Nolberto Solano in the most makeshift of back fours, they not only made a game of it, but at times threatened to take the initiative.
The in-form Cristiano Ronaldo and Giggs were a real handful throughout, while Wayne Rooney and Saha, until he was replaced by Park Ji-Sung on 36 minutes, kept the Magpies fully occupied.
But on the occasions that the men at the back were found wanting, United were met by a stubborn Shay Given, making his 400th appearance for the club.
The Irishman saw a ninth-minute Giggs shot fly across the face of goal, but had to make a fine block to deny Saha after the Welshman had played him in after 25 minutes.
Scott Parker, who was turning in a captain's performance in central midfield, headed a Saha effort over his own bar seconds later, but he forced Edwin van der Sar into a good save from distance on the half-hour.
However, it was the Magpies who took the lead after 33 minutes when Milner produced a stunning finish.
Kieron Dyer picked him out on the left and the England Under-21 international cut inside before blasting an unstoppable right-foot shot past Van der Sar.
Given had to keep out a point-blank Rooney effort seven minutes before half-time after United had broken at breathtaking speed, but he was finally beaten on 40 minutes when Scholes, who has a hat-trick at St James' to his name, ran onto Giggs' lay-off and drilled a low shot into the bottom corner.
Antoine Sibierski should have restored the home side's lead on the stroke of half-time when he headed a Solano cross straight at Van der Sar's legs, but the visitors responded immediately as Ronaldo shot wastefully over and Park clipped a post in injury time.
United needed only 22 seconds of the second half to get their noses in front and it was Scholes who did the damage once again, firing powerfully through Taylor's legs and past the unsighted Given to make it 2-1.
Darren Fletcher saw a shot blocked by Huntington and headed over from a Giggs cross in quick succession as Newcastle struggled to repel wave after wave of attacks.
However, the home side gradually started to make an impression going forward, although without ever troubling the visitors.
Vidic saw a 70th-minute header blocked, but then survived penalty appeals seconds later after Parker went down under his challenge.
However, Roeder's men were back in it within three minutes when Edgar took aim from 30 yards and saw his shot, with the help of a significant deflection off Scholes, nestle inside Van der Sar's left post.
United hammered away as the clock ran down, Rooney dragging a 78th-minute shot wide and Solano heading a Ronaldo effort off the line five minutes later.
However, the best chance of the closing stages fell to South Korean Park a minute from time when he controlled Ronaldo's cross at the far post, but lifted his shot over the bar.
Edgar thrilled with leveller
David Edgar described his equaliser for Newcastle in a 2-2 draw with Manchester United at St James' Park as 'unbelievable'.
The 19-year-old scored the Magpies' second, James Milner having put them 1-0
up before the Red Devils came back through a Paul Scholes brace.
It was the Canadian defender's first senior goal, and he told Sky Sports: 'It
was a big thing just to be called in and I took my chance. It was unbelievable,
playing in front of 52,000 Geordies is everything I dreamed of.'
Fellow goalscorer Milner added: 'We knew it was going to be tough - we were
disappointed at the weekend, we didn't deserve to lose 3-0 (at Everton) and
we've had another knock with injuries, losing Butty (Nicky Butt, who has an
ankle injury).
'We know the quality of the lads who have come in. It's never easy playing
against Manchester United.'
Red Devils boss Sir Alex Ferguson was full of praise for the hosts.
'We missed a stack of chances but you have to admire the character and spirit
of the Newcastle team. They deserved a point without question,' he told Sky
Sports 1.
'We had three cleared off the line so that was a bit unlucky. Then we went
and missed two or three good chances. When you are creating chances you hope you
are doing enough to win the match and we should have won the match.
'We didn't play as well as we have been because Newcastle didn't allow us to
play. I've nothing but praise for them.'
Striker Louis Saha was forced off with an injury and Ferguson said: 'It's a
groin injury so he may be out for a couple of weeks.'
Newcastle United 2 - 2 Manchester United
