Having dragged herself back into contention over the final three miles, the 32-year-old Scot was beaten by last year's runner-up, Joyce Chepchumba of Kenya, in the final 10 metres of an excruciating, extended sprint which had the crowds in The Mall roaring in astonishment.
The men's race was just as extraordinary as Antonio Pinto of Portugal moved from fifth to first in the final couple of miles, breasting the tape just two seconds ahead of Stefano Baldini, Italy's world half-marathon champion.
Richard Nerurkar, running a personal best of 2hr 08min 36sec, was the top male British runner home in fifth place, three places ahead of Paul Evans. It added up to the event's finest hour - or rather, two hours - since it began in 1981.
McColgan had seemed poised to become the first British woman to retain her title since Joyce Smith in 1982. After Germany's former European triathlon champion, Sonja Krolik, had seen a 50-second lead erased with three miles remaining, the former world 10,000 metres champion just managed to keep in touch with the two women - Chepchumba and Lidia Simon of Romania - who took over at the front.
Clearly responding to the crowd, she surged in the final mile to move through from fourth to first place, and the Romanian quickly dropped away.
McColgan's face assumed its characteristic Oriental look and her elbows swung ever more savagely from side to side as she attempted to put a conclusive distance between herself and the 27-year-old Kenyan.
As she accelerated into Birdcage Walk, and again on the final turn into The Mall, her flushed face and raised eyebrows expressed maximum endeavour. But Chepchumba, head waving from side to side in her efforts, refused to allow the Scot to escape.
When the Kenyan finally regained the lead there was no time for McColgan to respond. Passed, the Scot reached blindly to register her time on her watch - it was a personal best of 2:26:52 - before halting just past the line with her hands on her knees. The Kenyan, virtually in a state of collapse, vomited copiously before starting to acknowledge her finest achievement to date and a personal best of 2:26:51.
McColgan, her face pale and drawn beneath her extravagant top-knot, looked harrowed in the aftermath. "I'm very, very disappointed," she said. "I had so much support, and I feel as if I let the spectators down."
She felt her performance had been affected by a stomach problem which had prevented her taking on water in the later stages. "At 21 miles it was going through my head, 'Jeez, I'll be lucky if I finish in the first five'."
But she found the strength to contribute to what her coach, Grete Waitz, a legendary marathon runner of the 1980s and twice winner of this event, referred to as the most exciting marathon she had ever seen.
"Liz didn't realise Joyce was coming back at her until the final 10 metres, and then there was no time to react," Waitz said. "If she had realised with 50 metres to go it might have been different. With one mile to go I was sure it was going to be Liz because none of the other girls had her leg speed.
"If you can use the word mistake, she should probably not have gone before the final 400 metres. She made two moves, at 1,000 metres and 600 metres, but if she had waited until 400 to go...
"She is a tough cookie. That is her strength and also you can say a weakness. Because sometimes she is too tough on herself. That's where I come in."
McColgan was one of six women in the top 10 to set personal bests. The men did even better, with eight of the top 10 running faster than ever before as Pinto, a winner here in 1989 and runner-up two years ago, broke Steve Jones's 12-year-old course record of 2:08:16 with 2:07:55.
Before the 1995 event, Pinto responded foxily to the suggestion that Dionicio Ceron of Mexico and Australia's Steve Moneghetti would set off in search of the world best time. "I will wait for them and put them in my sack," he said.
But it was he who ran rashly, establishing a lead of more than a minute with five miles to go but being passed in the closing stages. "I tried to run differently from 1995, when I ran too fast in the first half," he said. He succeeded, astonishingly.
With a mile to go, the race seemed to be between Baldini and the Olympic champion, South Africa's Josiah Thugwane. But the Portuguese runner arrived like an imposter who had joined the race for the final stages, moving between the two astonished leaders and surging for home.
Baldini pushed him all the way to the line, but Pinto - despite a recent defeat by the Italian in the Lisbon half-marathon - felt confident his greater experience would tell.
Evans found it hard to believe that his time of 2:09:18 - his second- best ever - had only earned eighth place. "You would have thought that was worth a top three place beforehand," he said. Nerurkar found himself similarly powerless to stop Pinto. "He came by me in a flash," he said.
HOW THEY FINISHED
Men's top 10
1 A Pinto (Por) 2hr 07min 55sec
2 S Baldini (It) 2:07:57
3 J Thugwane (SA) 2:08:06
4 E Kimaiyo (Ken) 2:08:08
5 R Nerurkar (GB) 2:08:36
6 S Moneghetti (Aus) 2:08:45
7 L Peu (SA) 2:09:10
8 P Evans (GB) 2:09:18
9 J Garcia (Sp) 2:09:30
10 S Franke (Ger) 2:11:26
Women's top 10
1 J Chepchumba (Ken) 2:26:51
2 L McColgan (GB) 2:26:52
3 L Simon (Rom) 2:27:11
4 S Krolik (Ger) 2:28:02
5 R Burangulova (Rus) 2:28:07
6 M Machado (Por) 2:28:12
7 C McNamara (US) 2:28:18
8 R Kokowska (Pol) 2:28:21
9 Y Mazovka (Bela) 2:29:06
10 H Kimaiyo (Ken) 2:29:45Reuse content