The 46-year-old, who has raised more than £760,000 for charity, was cheered on by dozens of flag-waving supporters as she arrived at the Tate Modern art gallery at 4:10pm after running a marathon 26 miles from Windsor in Berkshire.
She began her epic run, cycle and swim in Edinburgh last Friday and told how she felt like a completely different woman since completing the “biblical” challenge.
Comedian Jo Brand announced that McCall had raised £760,026 in donations for causes around the world supported by Sport Relief.
Standing alongside her friend Brand and trainer Greg White, mother-of-three McCall dedicated her finish to “women and children everywhere” and admitted she was “trashed” after completing the arduous challenge.
She said: “I have no idea how I’m feeling, I don’t know whether I’m just so tired or so happy.
“I can’t quite believe it was me that did it. Greg was saying, ‘Last Friday … do you remember when we took the plane up to Edinburgh?’
“All I can remember is this really fearful woman. I was so frightened of what was ahead. It was a lot worse than, harder, much harder than I ever imagined it would be.
“But I did it and I don’t really know how I did it and I don’t know how I got here.”
Her final leg, a full marathon, began in flood-hit Windsor yesterday morning with BBC Radio 1 host Nick Grimshaw there to spur her on.
The former Big Brother host said she found the hardest part of the challenge was her swim across Lake Windermere in Cumbria.
“Windermere was my nemesis, but there were bits of every day where it got so dark and I have never cried as much as I’ve cried.
“I’m quite an emotional person. Since I’ve had children I’ve cried less. You’ve got to hold it together for the kids and on this journey I have wept like a baby at times every day and people have carried me through.”
She told how a member of her support team, Leon, carried her out of the water when her body temperature crashed dangerously close to hypothermia.
“What was amazing was I was worried about my health while I was in the water. I thought, ‘God, I am beginning to feel really cold and I’m so tired and I’m not going to make it’.”
She could remember someone grabbing her arm and telling her to stand up, but her body refused.
“I felt a man’s hand on my arm and I thought, ‘You’ve got me, I’m going to let go. I can’t stand up’.”
Her support team knew that the initial seconds after she left the lake would be critical, so they rushed her inside.
“But the recovery was so quick, it was weird. Within half an hour I was sat up in bed talking, they kept taking my temperature, waiting for my core temperature to warm up, and within an hour I was in the bath – and in an hour-and-a half I was on a bike.”