The 21-year-old took part in the challenge over the past five days in a bid to raise money for Red Nose Day.
But 50mph winds and choppy conditions on the lake meant he had to start a day later than planned, when he undertook a 65 mile cycle from Birmingham to Blenheim Palace, Oxfordshire.
He completed the final 50-mile stretch walking and cycling laps of Brands Hatch race track in Kent from 9am until 7pm on Friday.
Mr Monger, who began racing at the age of six but lost both of his legs in a racing accident three years ago, said: "The last few laps felt like a lifetime.
"It was the most exhausting experience of my life. At times I’ve never had to dig that deep. I never did this for me, it was about raising as much money as possible to change people’s lives.
The driver said this challenge has been "one of the toughest things I’ve ever had to do in my entire life”, adding that he couldn’t have got through it without everyone’s support.
“I want to say thank you for all your donations and your support as they’ll make such a difference to people’s lives and that’s what it’s all been about for me,” he said. “So please keeping donating if you can”.
A Comic Relief spokesman said: "Although Billy has finished the challenge, there is still a long way to go to raise money for Red Nose Day and help tackle issues including homelessness, hunger, domestic abuse, and mental health problems, all of which have been exacerbated by the Covid-19 pandemic, in the UK and around the world."
A documentary of his challenge will air on BBC One in March.