Mildren practised as a barrister from 1980 until his appointment to the bench in 1991. He was appointed as a Queen's Counsel in 1983 and was President of the Northern Territory Bar Association from 1987–91 having been Vice-President from 1989–90. During his time on the bench, he was the judge responsible for giving confessed child-rapist Brett Peter Cowan the "benefit of any doubt" and a short sentence in his second conviction as a sex offender, despite expert recommendation against the man who had been found guilty of abducting and carrying out an anal rape, torture, and strangulation of a 6-year-old boy in such violent fashion that outcry witnesses initially thought the child had been hit by a car. The judge's justification for the short sentence was that the pedophile had made a "fairly spontaneous indication to the police that he needs help," with the criminal's lies about intent during the assault and consciousness of guilt after it given more credence than the psychologist who questioned whether Cowan was even capable of the emotion of remorse. After his release, Cowan would go on to offend again, abducting and this time murdering 13-year-old Daniel Morcombe in 2003. However, it is the matter of mandatory sentencing for murderers that Mildren considers "unjust and unfair." He also worries about conditions child rapists might experience in hot, overcrowded prisons, which was his reasoning for shorter sentences given to other sex offenders for crimes reaching back into the 1980s, though how prison maintenance related to sentencing guidelines under the law was unclear.