Capped 15 times by Wales, Mitchell was in Brisbane to watch the Lions game last Saturday after touring Japan with his national team.
Mitchell, 27, was fined A$1,800 (£1,090) and also ordered by the Brisbane Magistrates Court to pay A$3,000 (£1,800) in compensation to the Brisbane resident.
Mitchell's companion, Richard Davies, was fined A$1,000 (£606) and ordered to pay A$2,000 (£1,012) in compensation for his part in the incident.
Local media said the victim had been knocked unconscious and published a photograph of him on websites showing heavy bruising around both eyes.
Both Mitchell and Davies had pleaded guilty after being charged with "assault occasioning bodily harm."