The Nottinghamshire born athlete, who is a double amputee, set off from John O'Groats in August and headed into Land's End on Monday.
The 37-year-old 200m sprinter, who began his career as a marathon runner, said the task had been five years in the planning.
He said the challenge had been "an epic journey".
"I'm really tired. It has been really tough the last couple of days," he said.
"Cornwall is really hilly and the energy levels are wacked at the moment. But I have had a great team around me who have supported me.
"It has been awesome to see people on the way and the support online as well."
Richard Runs Britain has raised £105,000 for charities Sarcoma UK and Scope, considerably less than his £1m target.
He asked people to run with him at certain points and was joined by several athletes and celebrities along the way.
Whitehead said he had been inspired by amputee and sarcoma cancer sufferer Terry Fox, who attempted to run across Canada in 1980.
He added: "Unfortunately he didn't reach his destination but he inspired me to start running myself and after last year's success at the Paralympics I got this once in a lifetime opportunity up and going, and here we are."
Before his challenge Whitehead said he had wanted to reach out to people of all ages and abilities and encourage people into sport as part of the legacy of the Olympics and Paralympics.
He was joined for the last two days of the journey by his wife and seven-month-old daughter.
Paralympic gold medallist Richard Whitehead has completed 40 marathons in 40 days.