The judges, whose ruling was confirmed by Robert Assael, a lawyer for one of the six, rejected their claims that they had immunity as they were protecting the president.
La Tribune newspaper reported that all six were ordered to pay fines, but that the punishment was suspended.
The appeal judges decided the six were outside their duty to defend Biya when they beat up Adrien Kreuze, a journalist for the Swiss public broadcasting network covering a demonstration outside a Geneva hotel.
The bodyguards injured Kreuze and took his phone and other items in the attack on June 26, 2019. The Swiss foreign ministry summoned Cameroon's ambassador over the incident.
After the ruling, Assael still insisted that his client had been protecting the president. He said there had been violence involving Biya opponents in Switzerland and Germany in the weeks ahead of the incident.
Bodyguards of Cameroon President Found Guilty of Attacking Journalist
FILE - In this March 22, 2018 photo, Cameroon President Paul Biya is seated with Chinese President Xi Jinping as they attend a signing ceremony at the Great Hall Of The People in Beijing.
GENEVA - Switzerland's appeal court has upheld guilty verdicts against six bodyguards of Cameroon's President Paul Biya for the 2019 assault of a journalist, the Keystone-ATS news agency reported Monday.