The 54-year-old had needed to run a double marathon on Sunday, finishing beneath a giant statue of Nelson Mandela in Johannesburg.
Izzard attempted a similar feat in South Africa in 2012 but had to pull out for health reasons.
It has been confirmed the comedian's endurance feat had raised more than Â£1.35m for the charity appeal.
"That was very, very tough," he told reporters as he swigged from a celebratory bottle of sparkling white wine.
The 27 marathons were intended to reflect the 27 years Mr Mandela had spent in prison before becoming South Africa's first black president.
"It's been the hardest thing I've ever done," said a "very tired" Izzard. "Thank you to everyone who has donated [and] don't do this at home."
Gary Lineker, marathon runner Paula Radcliffe and DJ Chris Moyles are among those who have tweeted their congratulations.

The 35-year-old has been released by Brighton & Hove Albion after they fell short in their bid for promotion.
"Maybe if I drop down a division, I'll not feature, but it's ifs, buts and maybes," the defender said of his international future.
"It is up to me to keep fit and keep playing and see where it takes me."
Greer, who won his 11th cap after starting in Saturday's 3-0 defeat by France in Metz, has no idea where he will be playing when Scotland start their World Cup qualifying campaign against Malta in September.
"Hopefully I'll get something and I'll be fit and ready for the qualifiers," said the centre-half who did not win his first cap until he turned 32.
"I am open to anything. I would like anything with a good opportunity where teams are pushing for something."
Greer had spent six years at Brighton after leaving Swindon Town but found himself out of Chris Hughton's starting XI after January.
Injuries led to a recall for their final game of the season against Middlesbrough, when a 1-1 draw denied Brighton automatic promotion, and he retained his place for the play-off semi-final defeat by Sheffield Wednesday.
The lengthy inactivity meant he was thankful for more game time against France despite being on the receiving end of a heavy defeat.
"It was very tough," said the former Clyde, Blackburn Rovers, Kilmarnock and Doncaster Rovers centre-half. "They are a very good team, got a lot of good players and we found it difficult at times.
"But it was good to go out there and get 90 minutes against some of the top players in the world, so it was interesting."
Scotland had lost 1-0 to Italy six days earlier and failed to fire a shot on target in either of the two friendlies against sides preparing for the Euro 2016 finals in France.
Greer was not one of those questioning the decision to play two very strong teams at the end of a long domestic season.
"Ultimately, as a footballer, you want to be playing out there against some of the best players in a great atmosphere because that's what you train for all season," he stressed.
"We've got some talented players coming through and I think it was really worthwhile for them as well.
"France are obviously at a different stage of their preparation going into the Euros and everybody's playing for places.
"We went out there and gave it everything, but we just fell short against a fantastic team."

A group of about 400 people, made up of the 15-year-old's friends, relatives and neighbours, took part in the Wear Pink for Paige walk on Saturday.
The walkers stopped at the spot in Clydebank where Paige's body was found and a single balloon was released.
John Leathem, 31, has appeared in court charged with murdering Paige.
He was fully committed for trial at Dumbarton Sheriff Court on Friday. He made no plea or declaration and was returned to custody.
Paige's body was found in a wooded area just off Great Western Road in Clydebank at lunchtime on 21 March.
The teenager was last seen near a deli in the town before setting off for her part-time hairdressing job.
Hundreds of people gathered for a memorial event a Clydebank park on Saturday 26 March.
The Titan Crane in Clydebank was also illuminated pink in memory of the teenager.
Comedian Eddie Izzard has completed his challenge in South Africa to run 27 marathons in 27 days for Sport Relief.
[NEXT_CONCEPT]
Gordon Greer has a double mission this summer - find a new club and one that is good enough to help him retain his place in the Scotland team.
[NEXT_CONCEPT]
Hundreds of people have paid their respects to schoolgirl Paige Doherty by walking through the streets of her home town wearing pink.