Farah trailed home in eighth place in the Virgin Money London Marathon - almost four minutes adrift of winner Wilson Kipsang, the world record holder from Kenya - in the toughest test of his career so far.
He failed to achieve his pre-race target of breaking the British record, his time of 2hr 8min 21sec short of Steve Jones' 29-year-old mark of 2hr 7min 13sec.
Farah knew his five global titles on the track would count for nothing against the strongest marathon field ever assembled, and he was dead right. The joy Farah experienced in London 20 months ago as he swept to double Olympic gold was absent.
The 31-year-old was never in contention, suffering in the last quarter of the race, the pain etched on his face as he was cheered on to the finish by fans lining the sun-drenched streets of the capital.
Farah was not helped by missing a drink stop shortly after the halfway mark, at that point running on his own rather than in a group, with his pacemaker too far in front to help.
Kipsang, in contrast, whose arrival in London had been delayed because his passport was stolen, broke the course record with his winning time of 2:04:29. His compatriot Stanley Biwott was second and Ethiopian defending champion Tsegaye Kebede third.
Farah branded his performance "a bad day at the office", but was adamant he would persevere with the marathon and that it had been the right choice to make his debut in the capital.
"I will be back, 100%," Farah said. "I'm not going to finish on a down. I wanted to give it a try. London is my city - this is where I grew up. It would have been wrong for me to do any other marathon.
"I want to be able to know I can run a great marathon as well as achieving medals on a track. I will do another marathon, but I don't know where, I don't know when. It could be next year, could be six months' time, could be a few years."
But Brendan Foster, the former Olympic 10,000m bronze medallist turned commentator, said he hoped Farah "stays on the track" for the time being to focus on the defence of his Olympic crowns in Rio de Janeiro in 2016.
Farah's build-up to the race had hardly been ideal, collapsing after crossing the finish line in second at the New York City Half-Marathon last month, although he stressed during the week he had suffered no ill-effects from that setback.
He received a huge cheer when introduced to the crowd at the start and certainly looked fired up, kissing the badge on his new GB kit.
He set off, as planned, with the second group, which dropped off the pace quickly as the leaders opened at speed.
From 10km Farah started to claw back time, moving through the field and reducing the gap to 38 seconds as he went through halfway in 1:03:08.
The pain really kicked in after the 25km mark, though. He dropped 64 seconds off the leaders at 30km and more than two minutes off by 35km as he slipped outside British record pace, eventually coming home 3:52 behind Kipsang.
Kipsang said he never considered Farah a threat, saying: "I did not fear anybody behind because I knew if you are really in top shape you should be in the leading group."
Chris Thompson, another debutant, was the second Briton home in an impressive 11th place, ahead of Olympic and world champion Stephen Kiprotich, in 2:11:19.
There was a Kenyan one-two in the women's race too, as Edna Kiplagat sprinted away from compatriot Florence Kiplagat down the Mall to win.
Kiplagat, the two-time reigning world champion, made up for finishing runner-up for the past two years in the capital to take the title in 2:20:21, with her namesake three seconds back.
Tirunesh Dibaba, the Ethiopian track great also stepping up to the marathon for the first time, had looked likely to challenge the pair for much of the race, but paid the price for dropping a water bottle and having to stop to pick it up. She finished in third, 14 seconds adrift.
Amy Whitehead was the first Briton home in 13th.
Wheelchair racer David Weir's bid for a record seventh title ended in disappointment as Switzerland's Marcel Hug edged him out by one second in a sprint for the line.
American Tatyana McFadden defended her women's crown, claiming a dominant victory a month after winning a cross-country skiing silver medal at the Winter Paralympics.