Despite being unknown on the political scene just six months ago, Adama Barrow comfortably defeated longtime leader Yahya Jammeh.
It is understood Mr Jammeh, who had previously vowed to rule for “a billion years”, will be releasing a statement to concede leadership of the West African nation.
Declaring Mr Barrow president for the next five years, Alieu Momarr Njai, head of Gambia’s electoral commission, called for peace and tolerance following the initial shock.
“There will be celebrations, there will be disappointment, but we all know we are all Gambia,” he said.
Mr Barrow won by 263,515 votes – 45% of the total – while Mr Jammeh finished second with 36%.
The father-of-five was thrust into the limelight following the jailing of top officials from the United Democratic Party in July, with eight opposition parties joining forces to put him forward as a unifying figure.
He now owns an estate agency but lived in Britain for three-and-a-half years when he was younger, working as a security guard for Argos in London, where he developed a love for Arsenal Football Club.
Gambians voted for him on Thursday by placing marbles into drums marked for each candidate.
Mr Jammeh had been confident of winning, saying his victory was all but assured by God while predicting “the biggest landslide in the history of the country”.
Omar Amadou Jallow, an opposition leader for the People’s Progressive Party, said the result will delight Gambians.
He said: “We are happy to be free. We are able to free the Gambian people from the clutches of dictatorship, and we are now going to make sure Gambia becomes a bastion of peace and coalition.
“Our foundation will be based on national reconciliation.”
Mr Jammeh came to power in a coup in 1994 and swept elections in 1996, 2001, 2006 and 2011 after a 2002 constitutional amendment removed presidential term limits.
Human rights groups have accused Mr Jammeh of ordering the deaths of countless political opponents as well as targeting journalists and homosexuals.