Parts of the capital Kingston were cordoned off in advance of the verdict and police patrols were stepped up.
Kartel, whose real name is Adidja Palmer, is one of the biggest names in Jamaican dancehall reggae.
The BBC’s Nick Davis in Kingston says that Kartel’s bleached skin – covered by tattoos – made him stand out from the crowd – and the “World Boss” as he called himself also often glorified violence in his music.
The 65-day trial was the longest running criminal hearing in the history of Jamaica’s circuit court system, local media reported.
Police cordoned off streets around the Supreme Court before Thursday’s session started, seeking to prevent any disruption by fans supporting the entertainer.
Shortly before the jury started to deliberate in the afternoon, about 200 people briefly broke through barricades at one intersection shouting “Free Kartel!”
The sentencing date is scheduled for 27 March and Kartel’s lawyers have indicated they will appeal against the verdict.
When Kartel was arrested in 2011, it was alleged he had been involved in two killings, but one of those cases was thrown out last year after key evidence went missing.
Prosecutors said that Williams was beaten to death at Kartel’s home in August 2011 after being lured there to account for two missing illegal guns.
A body has never been found but police testified that they had unearthed a text message from Kartel’s phone saying Williams had been chopped up to “mincemeat” so fine that his remains would never be found.
Defence lawyer Tom Tavares-Finson told jurors the prosecution’s case against Kartel was “dishonest” and “incompetent”, noting that witness statements and a compact disc with evidence saved on it had gone missing.
The AP news agency says that in a bizarre twist to the case, a male juror was arrested on Thursday evening on charges of attempting to bribe the jury foreman to free Kartel, who was convicted by a 10-1 majority verdict.