The 27-year-old was given a six-month prison sentence, suspended for two years, fined £1,180 and ordered to pay £1,966 in compensation to lawyer Cian Barry. He is now likely to be punished by his club, who confirmed in a statement he will now face disciplinary action.
"Exeter Rugby Club deeply regrets that one of its employees has faced court action in relation to their personal behaviour," said the statement.
"The club does not condone law breaking of any kind and will continue to stress with all of its employees the importance of acting as positive ambassadors - even in difficult and unwanted circumstances. Craig Mitchell wishes to express his remorse not only to the victim and those involved in the incident, but also to Exeter Rugby Club, the Welsh Rugby Union and his family."
Mitchell, who has 15 caps for Wales, had travelled to Australia to watch the Lions tour when the fight broke out on Sunday at the Normanby Hotel in Brisbane, following the tourists' first Test win against the host country.
Speaking at his sentencing on Friday at Brisbane Magistrates' Court, he apologised to Barry, saying: "My actions...were totally unacceptable and unforgivable."
The court was told Barry had been trying to smooth over an altercation between his friends and a group of Lions supporters.
Magistrate Anne Thacker told Mitchell he needed to be made an example of, saying: "It has to deter other Australians from this sort of behaviour at football matches."
Mitchell's lawyer John Cook said he could not say what implications it would have for his client's playing career, but he had received his passport back and was free to leave the country.
Mitchell admitted aggravated assault occasioning bodily harm.