Oshawa man given life sentence for 2024 double murder of upstairs neighbour and her ex-husband

Mark Gordon sat quietly in the prisoner’s box of a packed courtroom in Oshawa Tuesday as a judge sentenced the 33-year-old to two concurrent life sentences with no eligibility for parole for 21 years for the fatal shootings of a Bowmanville teacher and her ex-husband, a former Oshawa Generals hockey player.

Gordon pleaded guilty to two counts of second-degree murder in September 2025 admitting that on July 13, 2024, he fired multiple shots at 42-year-old Patrick Montgomery in front of the home where he lived on Glenforest Street in Oshawa, as his 12-year-old son watched on.

According to the facts, around noon that day, Gordon drove down the street, approached the house where Patrick lived and knocked on the door. The son recognized Gordon as the man who lived in the basement unit at his mother’s address.

Montgomery went outside and the two spoke in the driveway for roughly 15 minutes before the son looked outside to see Gordon pointing a handgun at his father. The boy heard his father say “please don’t” before Gordon fired several shots at his father.

“As he left, he looked at (the boy) who put his hands up, believing he was going to be shot,” the facts state.

Gordon then got into his truck and drove 400 metres down Glenforest Street to where 42-year-old Andrieana Montgomery, Patrick’s ex-wife and the mother of their two children, rented the upstairs unit of a home.

Gordon exited his truck and began firing at Andrieana, who was standing at the sink washing dishes while looking out of the front window of the home. He fired six shots from outside and then entered the residence and fired a further three shots. Andrieana was shot four times including once in the head.

Gordon rented the basement unit of the home with his partner and her young son. Court heard Gordon and Andrieana did not have an amicable relationship.

In the months preceding July 13, 2024, they argued with one another about noise coming from each other’s unit. During one such argument in the winter of 2023, Gordon told Andrieana that he had a gun.

He was, in fact, the lawful owner of a Glock 17 gen 5 semi-automatic 9 mm handgun and some rifles. He stored the firearms in a metal safe in the unit. He had a valid possession and acquisition license for the firearms.

Andrieana felt threatened when Gordon advised he had a gun. She complained to their shared landlord about the excessive noise coming from the basement unit and advised him of her concern that Gordon had a firearm. Andriena, who lived with her two sons, aged 10 and 12, told her children, family members and friends about the conflict with Gordon and that he possessed a firearm.

The facts state that Andriena told her children, who lived with her, that Gordon was not nice and not to trust him and she was afraid of him.

On May 8, 2024, Andrieana contacted the City of Oshawa and reported that the basement of the house did not meet the legal requirements for a basement suite. The City of Oshawa contacted the landlord about the violations and provided options for addressing the issue.

The landlord decided to evict Gordon, who lived with his girlfriend and her three-year-son son in the basement unit. Gordon was served with a notice of eviction on July 12, 2024.

The following day, he shot the Montgomerys in what Madam Justice Michelle Fuerst called an extremely violent attack saying Gordon knew he was leaving two children without parents when he shot them both.

“The callousness of his decision to act as he did in the face of that knowledge is immense,” Fuerst told the court.

According to a presentence report, Gordon said as the relationship between him and his upstairs neighbour continued to deteriorate, his perspective in his own words was that Andrieana Montgomery was ‘bullying him” and his “disliked being bullied.”

He also told the author of the presentence report that the eviction notice was the catalyst for the murders, it was the “last straw for him.”

Gordon was arrested at the scene on July 13, 2024 and has been in custody ever since. He apologized in court saying that he was wrong for what he did and that he is the sole person to blame and that he regrets everything.

The judge found it extremely aggravating that Gordon fired shots at victims who were defenceless and Patrick was executed in front of his son.