Matthew Rees, who was competing as part of the Swansea Harriers running club, stopped metres short of the finishing line to help David Wyeth of Chorlton Runners finish his race.
Wyeth's legs began to buckle as he approached the finishing line as the gruelling 26-mile course began to take its toll.
Mr Rees told the Press Association: "I took the final corner thinking 'right, it's nearly done, time to sprint', and I saw this guy and his legs just crumbled below him.
"I saw him try to stand up again and his legs just went down again, and I thought 'this is more important, getting him across the line is more important than shaving a few seconds off my time'.
"I went over to try and help him and every time he tried to get up he just fell down again and again, so I just tried to cheer him on, picked him up and said: 'Come on, we can do this'.
"He was really grateful, but he wasn't very coherent, he was just like 'I have to finish, I have to finish' and I said 'you will finish, you will get there, come on let's do this', but every time he tried to move he would just fall again so it was important to guide him."
With Wyeth clearly struggling to maintain his footing, Rees abandoned his own race to help carry the Chortlon runner towards the line.
Wyeth received immediate medical attention from London paramedics before being taking away for further attention.
"My calf started to cramp really early on, so I just decided 'right I've just got to make it to the finish line' and so when I saw the guy on the floor it sort of brought the feelings I'd had the whole marathon to me," said Rees.
"I wanted just to get to finishing line and that's all he wanted - so we could just do it together.
"We've got the exact same emotions but in different ways, he can't walk and I have just had an awful race, I just want to get to the finish line and so does he.
"This is what the marathon is about - it's about people - it's for everyone. Moments like this make it worth it. I'm just glad he's okay."
Rees was hailed as a 'legend' and 'an amazing bloke' but television viewers with the act of sportsmanship symbolising the sprit of the London Marathon.
"Matthew Rees, of @SwanHarrierDev, you've just encompassed everything that's so special about the #LondonMarathon. We salute you," read a post from the London Marathon's official Twitter account.
"No.1574 Swansea Harriers - Matthew Rees .. you're a hero for stopping for that guy in trouble when so many ran past," wrote another user on Twitter.