He claimed to have invented his own herbal HIV 'cure', and was once accused of force-feeding hallucinogenic potions to an entire village of people he thought had bewitched his aunt.
The man who styled himself 'Excellency Sheikh Professor Doctor President' has been defeated by Mr Barrow, who spent his early years tackling shoplifters at the Argos store.
Now a successful estate agent in his home country, the quietly-spoken Mr Barrow was one of the few Gambians brave enough to take Mr Jammeh on in the contest.
Mr Barrow's victory comes despite Mr Jammeh's ministers making fun of him over his days doing low-level security work in London.
At a pro-government rally ahead of the elections, Sheriff Bojang, the Gambian information minister, told the crowd sarcastically: 'We have heard that Mr Barrow worked as a security guard in... what is it... this shop called Argos in Britain?'
Lamin Cham, Mr Barrow's spokesman, said that the opposition leader would go to State House in the capital, Banjul, to be officially declared victor at 5pm today.
'It's game over,' he told the MailOnline.
Like Zimbabwe's Robert Mugabe, Mr Jammeh describes any criticism of him by Western governments as 'neo-colonial' lecturing.
He is particularly scathing of Britain, and at a recent campaign rally attended by the Mailonline, declared that Britain had done "nothing" for Gambia during "400 years of colonialism".
His claims were dismissed as posturing by Mr Barrow, who said that he owed his career success to his tough years in Britain, where he lived and studied property management between 1998 and 2002.
During that time he paid his rent with security jobs in offices, music festivals and high street stores, once tackling a shoplifter at Argos who was jailed for six months.
He also lived on a crime-plagued housing estate in Kidbrooke in south-east London, which has since been demolished.
"My time in Britain taught me the importance of working hard and good time-keeping, and both those things helped me a lot when I went back home," said Mr Barrow, 51.
"People think life is easy in Europe, but it can be a tough place to make your way in as a visitor."
A nation of just 1.9m people, tiny Gambia is best known in the West as a winter sun destination, attracting 100,000 foreign tourists every year to its pristine Atlantic beaches.
Britons make up around half the visitors, with thousands also living there in retirement.
Since taking power in a military coup in 1994, 51-year-old Mr Jammeh has won four elections, most of which have been marred by accusations of cheating.
Critics say he ruled by decree, with anyone who disagrees with him liable to end up in the notorious Mile Two prison, which is not far from the main tourist beaches in the capital, Banjul.
In April, several opposition figures in Mr Barrow's United Democratic Party were thrown in jail, and another two fatally beaten in custody
