Matthew Charles Johnson (born 9 August 1973) is an Australian criminal who is known for his extreme acts of violence, including the murder of Carl Williams, a drug dealer and serial killer.

Johnson was raised in Dandenong, Melbourne, Victoria; during his childhood, he experienced familial challenges and was expelled from school in year seven.

Johnson has a decades-long criminal history that includes convictions for burglary, armed robbery, serious violent offences, and murder.

His best-known crime is the murder of Williams in 2010, which he committed using the stem of an exercise-bike seat in Barwon Prison's Acacia Unit, where both men were imprisoned.

During his trial for the murder, Johnson claimed the act was in self-defence but he was found guilty and sentenced to life imprisonment with a non-parole period of 32 years.

[3] Starting at age 15, Matthew Charles Johnson attended multiple youth-training centres and was jailed for burglary, motor vehicle theft, and weapons offences.

As a teenager, he served three separate terms at HM Prison Pentridge,[1] and at 18, he was imprisoned for serious theft and drug offences.

[4] In his 20s, after his release from Pentridge, Johnson secured employment as a concreter and was able to stay out of trouble until he suffered a workplace injury.

[1] In September the following year, Johnson was tried for an attack on contract killer Gregory Brazel in Barwon Prison's Acacia unit.

The trial jury found Johnson and two others guilty of seriously injuring Brazel with the stem of a bike seat and other weapons.

Johnson pointed a loaded gun at one of the occupants before the pair, who were intoxicated with drugs, realised they had robbed the wrong house.

It was alleged on 22 May 2007, Johnson and his co-accused Mark Morgan shot and killed Conyers over a A$50 drug debt at the victim's home in Berwick.

Using a metal bar from the seat of an exercise bike, Johnson struck Williams on the head, causing him to fall off the chair on which he was sitting.

[16] The judge Lex Lasry called this claim "fanciful" and during sentencing, he found the more-likely reason for the killing was that Williams was assisting police with their case against Dale and Rodney Collins for the Hodson murders.

[17] In 2021, Johnson launched an appeal against his conviction for murdering Williams based on supposed findings in the Royal Commission into the Management of Police Informants.