Kenneth Bassarath, 64, killed Sergio Mendes when he swung the bat at him as daughter Celosia and her two young sons cowered in a bedroom.
She had locked herself in with them and telephoned her father for help as her husband, who she had never seen so angry, ranted and banged at the door.
Bassarath managed to wrench the cricket bat from his son-in-law when he arrived and took a swing with it to fend him off.
He feared that Mr Mendes, a 32-year-old supermarket worker, had armed himself with a knife as he had done so before, Old Bailey jurors heard.
Bassarath said he did not intend to strike his son-in-law, but the blow caught him on the head, and he fell to the ground.
The father-in-law then stood above him and was heard by a neighbour to say: "He's beaten my daughter. Call the police."
When officers arrived it was Mr Bassarath who was arrested and Mr Mendes was taken to hospital, where he died three days later from brain injuries.
But Trinidad-born Bassarath, of Hackney, east London, denied murder, claiming he was acting in self-defence.
His daughter wept as she told jurors: "It was an accident. We all loved each other and it is not fair what's happened to us."
The jury acquitted him of murder but still found him guilty of manslaughter.
He was granted bail but faces a likely custodial term when he returns for sentence on 16 December.
Bassarath had gone round to the address in March this year with his son Kendal after receiving the call from his daughter while he was in bed at his home nearby, William Boyce QC, prosecuting, told jurors.
"The defendant believed that the deceased was unstable because of previous occasions he had had to go round to his daughter's flat to calm down a volatile situation, on one occasion taking a knife from Sergio," said Mr Boyce.
Bassarath said that when they arrived, Mr Mendes chased Kendal with a metal pole before re-arming himself with the bat.
After his arrest he explained how he had taken the bat from him, and said: "I was just defending my son and daughter."
He told the court that he felt threatened when he saw his son-in-law coming towards him.
"I raised the bat because I thought maybe he had a knife and I swung it, not to hit him, just to fend him away," he said.
"It so happened that he came and I struck him on the head.
Bassarath said he had been "pals" with Mr Mendes and was even teaching him to play cricket. Immediately after hitting him he was "filled with remorse and regret", he said.
His daughter Celosia told the jury how she had recently booked a family holiday to Florida and was hoping to sort out her marital problems when the violent row that led to her husband's death begun.
She described how he had tried to restrain her by pinning her down on the sofa.
"I was really scared and at that time I just started crying," she said.
She broke free and scrambled upstairs but he pulled her down by the ankle so she pushed him backwards.
"He got really really angry. I have never seen him that angry in all my ten years of being married to him.
"He said 'are you trying to break my neck?' I got really scared. I ran up the stairs and I locked myself in the room where the children were sleeping.
"I was locked in the room. My six-year-old was asleep but the three-year-old baby was awake.
"He was drinking his milk and his eyes were open and he looked a bit scared.
"Sergio came up the stairs and started banging on the door to be let in. He said to me 'open this door, open this effing door, open this door now'.
"He said 'open this effing door otherwise I am going to break it down, you can't keep my kids away from me'.
"He kept banging on the door. The little one was scared - I think the baby saw that I was scared."
She called her father to come round and also dialled 999 for the police, telling the operator that her husband was trying to beat her up.
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