The vehicle was owned by a 40-year-old man who was washing it outside his New Farm residence at the time.
With the keys already in the ignition, Hooper attempted to start the car before the man tried to pull the ex-Olympic boxer out of the driver's seat and they struggled.
They both fell to the ground and the man - who did not know Hooper - yelled: "Call the police".
Hooper broke free and punched the man in the head several times.
The man was screaming for help and trying to protect himself, Brisbane District Court was told.
Hooper struck the 40-year-old a few more times when he refused to give him his phone.
A bystander approached and asked the boxing champ to stop before Hooper - who was unlicensed - drove away in the car.
The man suffered a bleeding nose and lip along with face bruising and was treated at hospital.
Hooper's fingerprints were later found in the car, which was dumped on the Sunshine Coast after sustaining almost $5000 worth of damage.
Ex-WBO and IBF light heavyweight champion Hooper had previously been diagnosed with schizophrenia and was "actively psychotic" at the time of the July 2022 offence, the court heard.
A psychological report said Hooper believed at the time that the healing centre he was attending was "evil" and that he was being pursued by another inpatient who planned to attack him.
"The psychologist opines and I accept that there is a direct causal link between your mental state at the time of the offending and this offence," Judge Deborah Holliday said.
"You had previously been diagnosed with schizophrenia, you had been taken off your medication and you were actively psychotic at the time of this offence.
"You believed that you were being pursued by another inpatient and that it is an explanation for the offending."
Judge Holliday said she was satisfied Hooper's moral culpability was reduced.
Hooper started using methamphetamine at about 18 before going through a period of stability and excelling at boxing, the court heard.
He contested the 2012 London Olympics and 2010 Delhi Commonwealth Games for Australia, turning pro in 2013 and featuring on the Jeff Horn-Manny Pacquiao undercard.
He compiled a 14-1 win-loss record featuring nine knockout victories over a five-year professional career.
However, he started using the drug again at 24 which coincided with his first period of psychosis, the court heard.
His return to drug use was "the catalyst for your psychotic illness", the psychological report said.
Hooper likely meets the criteria for untreated PTSD, the report said.
He now regrets his offending, has a stable mental state and is taking medication, the court heard.
Hooper, 31, on Friday pleaded guilty to robbery with personal violence and was sentenced to 27 months in jail, with immediate parole release.
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