The Somalia-born British track and field athlete was heading to Portland, Oregon to spend the festive season with his family when he was quizzed by airport chiefs.
Farah admits it is not the first time he's been held at customs, as his passport often raises questions about his background.
He tells Britain's The Sun newspaper, ""I couldn't believe it. Because of my Somali origin I get detained every time I come through U.S. Customs. This time I even got my medals out to show who I am, but they wouldn't have it.""
The sports ace, who is sponsored by athletic brand Nike, also reveals he experienced issues obtaining a U.S. visa - and his trainer Alberto Salazar had to use his connections in order for Farah to enter the country from Canada.
He adds, ""When we tried to get a residency visa, Nike obviously signed it all off and we thought it was going to be straightforward. We were in Portland on a tourist visa so had to leave and re-enter the country as a resident.
""So we flew to Toronto to stay for a few days, then come back in. But when we were there we got a letter telling us we're under investigation as a terrorist threat and we would have to stay away for 90 days.
""We had only packed four days' worth of clothes. We didn't know what to do. As luck would have it Alberto has a friend who works for the FBI. This guy happens to be a massive running fan, knew exactly who I was and got it sorted then and there. God knows what would have happened if he didn't. We'd probably still be in Toronto.""