Michael Lane arrives by van at Lewes Crown Court, where he is accused of the murder of Shana Grice, whose body was discovered at her house in Chrisdory Road, Brighton. Photo: Gareth Fuller/PA Wire
There were claps and cheers in court when the jury returned its verdict after just over two hours of deliberation.
In the months before she died, Lane had been stalking Miss Grice, the trial heard.
He put a tracker device on her car and received notifications via a phone app every time it moved.
Prosecutor Philip Bennetts QC said Lane became "obsessed" with Miss Grice after forming a relationship with her in 2015 while they both worked at Brighton Fire Alarms.
He told jurors: "During their relationship, he stalked her. He put a tracker on her car and followed her movements. That obsession with her translated into killing her. He would not allow anyone else to be with her."
Miss Grice complained about her tyres repeatedly being let down, of receiving heavy breathing phone calls from withheld numbers and of Lane following her.
Lane also targeted Mr Cooke by placing a note on his car which read: "Dear Ash, Shana has and always will cheat on you. Happy New Year."
Sussex Police will now face questions after it emerged that officers were called at least four times about Lane in the months before the murder.