She admitted stabbing him in the neck and cutting his throat and burying him in the backyard.
She said she "just snapped" during an altercation but did not mean to kill him.
Berry's remains were found two weeks after he died - when Joyce went to the police and confessed she had "manslaughtered" him.
She was charged with murder and has been on trial in the High Court at Christchurch before Justice Jonathan Eaton and a jury.
While Joyce, 56, conceded she killed Berry she pleaded not guilty to the murder charge.
The jury heard evidence from more than 30 witnesses including Joyce herself.
The couple's troubled relationship was also described by various family, friends, neighbours and others connected to them.
Joyce had a string of convictions for assaulting Berry and had spent time in prison as a result.
When he died, Berry had a protection order in place against Joyce.
Details of the day Joyce handed herself to the police including an almost two-hour video of her "confession" interview with detectives were also revealed in court.
Earlier on Monday, the Crown and defence gave their final statements to the jury before Justice Eaton summed up the case and sent the jury out to deliberate.
When the verdict was given in court just before 8pm Joyce showed no emotion
Berry's family, including his brother David, sister Joanna and nieces and nephew - who have been in court for the majority of the trial - wept as the verdict was read and held hands.
They issued a brief statement on Monday evening.
"We would just like to thank everyone that has been involved with the case and the many hours put in to reach the verdict today," they said.
"No one has the right to take a son, brother, brother-in-law, uncle or friend and Martin was that and much much more to us all.
"As a family, we are pleased that Martin's voice has been heard and that the right conclusion to this sad occasion is over."
Joyce was remanded in custody until sentencing, which will take place in the High Court at Christchurch next month.
Joyce's criminal history can now be reported.
In 2014 she was sentenced to 12 months' supervision and ordered to complete 200 hours of community work for attempted arson and wilful damage
Stuff reported that Joyce, then 49 and living in Tokomaru, tried to set fire to her neighbour's boat while she was intoxicated.
A summary of facts read in the Palmerston North District Court stated Joyce did not get along with her neighbours and after consuming "a large volume of alcohol" she went to their property when they were not home and poured petrol all over their boat.
Before Joyce could do anything else a fire siren went off and frightened her.
