Mr Mahon denied murdering his stepson in May 2013
David Mahon has been found guilty of killing his stepson Dean Fitzpatrick outside his apartment at Northern Cross, Dublin 17 in May 2013.
The 45-year-old, who is married to Dean's mother Audrey, was charged with murder but convicted of the lesser charge of manslaughter.
Just before the jurors returned with their verdict, Mahon walked over to his wife Audrey, gave her a bank card and squeezed her hand.
He then took his seat in the dock and moments later, he became the convicted killer of her son Dean Fitzpatrick.
With both hands resting on top of his walking stick, the 45-year-old gave very little reaction.
He just stared straight ahead. Audrey wiped a tear from her eye.
Mr Mahon was charged with murder, but the jurors found him guilty of the lesser charge of manslaughter.
He said he did not mean to kill the 23-year-old. He told gardaí he just walked into the knife during an argument on May 25th 2013.
He even suggested he may have wanted to walk into it because of his history of self-harm.
The 45-year-old was remanded in custody and tried to make his way over to Audrey - but was stopped by a prison officer and escorted out of the court room.
The court heard more than one victim impact statement may be required for his sentence hearing, to reflect the views of different sides of the Fitzpatrick family.
He is due to be sentenced later this month.
Audrey left court after the verdict.
VIDEO Audrey Fitzpatrick leaving court after her husband Dave Mahon was convicted of killing her son Dean pic.twitter.com/bNrBEeaMmF— Frank Greaney (@FrankGreaney) May 6, 2016