But he has found himself in the midst of a media frenzy after claims in a CBS 60 Minutes programme that his account is riddled with inaccuracies.
The report dissected the financial affairs of the Central Asia Institute (CAI), the charity he set up, but it also raised serious doubts over whether the organisation really did build all the schools it has taken credit for. Mr Mortenson denies the allegations.
The BBC investigated a few areas of Afghanistan and Pakistan to test some of the central claims made in the programme.
The mountainous Gilgit-Baltistan region in northern Pakistan hosts areas of breathtaking beauty and profound deprivation. It was here, in 1993, that Mr Mortenson says he stumbled upon the remote, farming community of Korphe, whose hospitality sparked off a school-building programme across Central Asia.
This story is disputed by 60 Minutes and his school-building record is also questioned.
But Imran Nadeem Shigri, a politician from the neighbouring Shigar valley in Baltistan, where the CAI says it has established a number of schools, says "he did some pretty useful work in our region and brought in some much needed funds."
Mr Shigri is not a Mortenson cheerleader - he is a man Mr Mortenson considered to be an opponent and allied to a cleric who issued a fatwa against Mr Mortenson at the start of his school-building mission.
Despite this, Mr Shigri says that the CAI has built even more schools in the region than it shows on its website. He is conducting a government survey of CAI schools in the area and thinks this is because it was only counting schools it managed, not those it helped build.
"During my tenure in the government, we took over at least seven CAI schools. The buildings and the equipment was there. The government provided the teachers. It was good for the government as it got free building and equipment. It was good for the CAI as their projects were made viable," he said.
In the parts of Afghanistan the BBC investigated, the picture was patchier.
In the mountainous north-eastern province of Badakshan, the educational authority says that all the schools listed on the CAI website do exist. But, in its view, the schools were not built in a satisfactory manner, and run the risk of collapsing, putting students at risk.
The authority says it was not consulted when the schools were being constructed and cites the example of Baharak Primary School, which it says was built too close to the river and is at risk of flooding.
It says the CAI school-building programme in Badakshan represents a waste of money.
But supporters of the CAI point dispute this, pointing out that local officials in Badakshan actually gave the organisation an award late last year in appreciation of all the schools it had built in the area.
In the insurgency-wracked province of Kunar, many of the schools appear to be in districts beyond government control. The educational authority there said that only two of the schools mentioned on the CAI website - Samarak and Suna Gal Primary School - could be credited to the organisation.
One other building listed as a school, Shir Gul, appears to be a clinic built by the CAI. It said that all the other schools were built by Nato's Provincial Reconstruction Teams or by a charity, the Swedish Committee for Afghanistan.
However a local education ministry official in Kunar insisted that the CAI's work is appreciated and that the organisation had provided buildings, pens, furniture and notebooks.
The CAI in Afghanistan has also strongly disputed any suggestion of wrong-doing. Its director of operations Wakil Karimi told the BBC that four schools in Kunar have been completed and are being used.
Mr Karimi said that five other schools remain incomplete because of the threat posed by Taliban militants and because of official corruption. He said that central government officials seldom visited the areas where new schools were being built and were in some cases demanding bribes before they gave permission for schools to start operating.
The Afghan education ministry has strongly denied these claims. In a statement to the BBC it said that CAI's work in Afghanistan "did not meet expectations".
But Mr Karimi insisted this was not the correct conclusion.
"Just because we don't put signs up doesn't mean we don't run a school," he said. "I am ready to take any member of the press so that they can see it for themselves."
Yet despite the CAI's protestations, few dispute that the local operation of Mr Mortenson's charity sometimes appears to take on a chaotic character.
Mr Shigri says that in the case of Gilgit-Baltistan, although Mr Mortenson brought in a lot of funds, he also failed to develop an efficient administrative system.
"He tended to use those funds through people that he had personally known and trusted. So in the end the CAI was being billed twice or three times the amount that was actually being spent on its operations," he said.
Local disputes could also account for some of the accusations levelled at Mr Mortenson. Mr Nadeem points to a rift between Mr Mortenson and the local team running the CAI in Gilgit-Baltistan.
"Over the last six months or so, there has been a propaganda campaign being run by the locals saying Mr Mortenson has written some things about Islam in his book which are objectionable. I think the locals want to prevent Mr Mortenson from laying claim to the CAI assets in Pakistan which have for some time been appropriated by them," he says.
When describing the Gilgit-Baltistan school-building programme, Mr Mortenson told Outside Online magazine that he had devolved responsibility to a local manager who, he says, ended up effectively cheating the organisation.
Mr Mortenson also told the magazine that management was not a strength of his.
"We've already been working very hard to bring changes to the way we work. I've also said I am not a good manager," he said.
In Afghanistan, Kabul province's educational authority also said that CAI work was not as effective as it should have been.
But this could partly be due to the tortuous nature of Afghan and Pakistani bureaucracy - the operations of a whole range of educational charities in the region could well expose inefficiencies if placed under a similar microscope.
And the workings of remote tribal and feudal cultures can also explain discrepancies. In remote areas of Pakistan, "ghost schools" can arise - where locally influential people provide land for government-funded school buildings which they later use for their own purposes.
What the locals say
Local leaders in some of the remote and poverty-stricken communities, where Greg Mortenson has established a presence, have voiced their support for him.
In Badakshan, despite the provincial authority condemning the CAI school-building programme there, one UN worker from an area in which a school had been built said the locals really appreciated it - hundreds of children attend.
"Imagine if the money had been given to the Afghan government or contractors - it would not have been built!" the worker, who wished to remain unnamed, said.
Two tribal elders in Kunar, one in the provincial capital, and one in the district of Naray where one school is said to have been built, knew Mr Mortenson - they said people were very happy with the work he had done.
Mr Mortenson always wore Afghan clothes when in the country and his knowledge of Afghan customs and traditions impressed many of the locals around him.
By M Ilyas Khan and Bilal Sarwary..BBC