Switzerland tried the case under the principle of universal jurisdiction, which allows countries to prosecute people for crimes that took place elsewhere.
Sonko is the highest-ranking government official ever to be prosecuted under this principle in Europe.
Philip Grant, head of the organisation that filed the complaint leading to Sonko’s arrest, said the case sends a “resounding message against impunity”.
“Minister-level perpetrators are now within reach of justice,” the Trial International director added.
Swiss investigators travelled to The Gambia and interviewed dozens of alleged victims and witnesses for the trial, which began in January this year.
Under Mr Jammeh, who was in power from 1996 until 2016, The Gambia was characterised by “widespread abuses, including forced disappearances, and extrajudicial killings”, according to Human Rights Watch
Sonko was seen as Mr Jammeh’s right-hand man, his role as interior minister putting him in charge of the security services, including, allegedly, a sinister paramilitary group known as “the Junglers”.
But in 2016, shortly before Mr Jammeh lost power, Sonko fled to Switzerland, where he claimed asylum.
In January 2017, he was arrested by authorities there.
Along with Switzerland, other countries are bringing cases against former members of Mr Jammeh’s regime.
In October, Germany handed a life sentence for crimes against humanity to Bai Lowe, a one-time member of “the Junglers”.
And in September this year, a court in the US state of Colorado will try an alleged former member of the same group.
Although The Gambia has created its own transitional justice process to address abuses committed under Mr Jammeh’s rule, human rights groups say its work has so far been very slow.