Simon Harris, 55, carried out sex attacks on boys in the town of Gilgil, between 1996 and 2013.
Harris, of Pudleston, near Leominster, was head of charity VAE, which placed volunteers in Kenyan schools.
He was convicted of eight counts of indecent and sexual assault, at Birmingham Crown Court in December.
The case is believed to be the first using legislation allowing British nationals to be tried in this country for crimes committed abroad, if it is also an offence in that country.
The jury also convicted him of four counts of possessing indecent images of children.
He was, however, cleared of 10 further charges, including rape.
Harris, a former classics teacher, had preyed on "very small children who have no families, nobody to look after them... sheltering in doorways" with the promise of food and shelter, the jury heard.
Judge Phillip Parker QC described him as "intelligent" and "charismatic", giving him "a veneer of respectability", which enabled him to design his life to be close to boys.
In Kenya, Harris had assumed a "hallowed" position, accountable to no-one, that allowed him to abuse street children who were amongst the most vulnerable in the world, the judge said.
"It had been urged upon me that I should give you credit for all the charitable work you introduced to Kenya, but I am afraid, when your so-called charity work is the vehicle for abuse, I cannot buy into this concept," the judge said.
He described Harris as "a significant risk" to young boys and his Kenyan victims had been left "used, degraded, and humiliated".
Travel ban lifted
"The mental scars will almost certainly never heal," Judge Parker said.
Harris is expected to serve half his sentence on licence and was also handed a life-long sexual offences prevention order.
The abuse came to light after evidence was passed on by a Channel 4 documentary team making a film about the plight of Gilgil's street children.
Officers from the National Crime Agency, who travelled to Kenya in a bid to trace Harris's victims, described him as "one of the most prolific child sex offenders" they had ever encountered.
It emerged during his trial that Harris had been banned from travelling abroad after a 2009 conviction for possession of indecent images of children, for which he served 15 months in jail.
However, the jury heard he successfully appealed to have the ban lifted, allowing him to travel back and forth to Kenya, where he abused more boys, until he was eventually arrested back in the UK in June 2013, by West Mercia Police.
Speaking after the conviction, officers said they had identified a number of potential victims during their inquiry but believe Harris could have abused hundreds of boys.
"The convictions and today's sentence sends an important message to people who have been the victim of crimes such as these to come forward, because time and geography is no barrier to justice," Det Ch Insp Damian Barrett said.
"The emotional impact on the victims of the abhorrent actions of this man is impossible to quantify."
Harris's trial is thought to be the first involving the use of live video link technology from Africa to hear evidence.
Harris had denied all the offences in Kenya, but before his trial got under way he admitted six offences of indecent assault against three boys, aged between 13 and 14, when he was a teacher at Shebbear College, Devon in the 1980s.