Sir Mo's PE teacher, Alan Watkinson, noticed a transformation in the young boy when he hit the athletics track.
"The only language he seemed to understand was the language of PE and sport," he says.
Sir Mo says sport was a lifeline for him as "the only thing I could do to get away from this [living situation] was to get out and run".
He eventually confided in Mr Watkinson about his true identity, his background, and the family he was being forced to work for.
The PE teacher contacted social services and helped Sir Mo to be fostered by another Somali family.
"I still missed my real family, but from that moment everything got better," Sir Mo says.
"I felt like a lot of stuff was lifted off my shoulders, and I felt like me. That's when Mo came out - the real Mo."
Sir Mo began making a name for himself as an athlete and aged 14 he was invited to compete for English schools at a race in Latvia - but he didn't have any travel documents.
Mr Watkinson helped him apply for British citizenship under the name Mohamed Farah, which was granted in July 2000.
In the documentary, barrister Alan Briddock tells Sir Mo his nationality was technically "obtained by fraud or misrepresentations".
Legally, the government can remove a person's British nationality if their citizenship was obtained through fraud.
