He still denies the offending, saying police got the wrong man for the Riverhead attack, and that the 1992 incident was a case of consensual sex.
During the trial the Crown was also able to call "propensity evidence" - meaning they could tell the jury about Mitchell's prior conviction for raping a woman in 1984.
READ MORE:
• Colin Jack Mitchell 'like a shadow' over 1984 rape victim's life
Jurors are not usually given information about the previous criminal offending of a person on trial, but in Mitchell's case the Crown argued it was necessary, that it showed his propensity to act in a particular way.
Justice Sally Fitzgerald imposed a sentence of preventive detention - an indeterminate prison sentence where, even if prisoners are released on parole, they remain under the management and monitoring of Corrections for the rest of their life.
She said the sentence had to reflect the seriousness of offending, denounce his actions and protect the community from him in future.
Justice Fitzgerald said Mitchell's offending was "disturbing".
Soon after he was imprisoned, Mitchell filed an appeal against his sentence and his convictions.
The grounds for his appeal was that he should have had separate trials for the Riverhead attack and 1992 rape and that the details of the 1984 rape should not have been admitted as evidence.
Both issues were tabled before the court well before the trial.
Mitchell's appeal was heard in Auckland today by Justice Stephen Kos, Justice Mark Woolford and Justice Rachel Dunningham.
Defence lawyer Mark Ryan said the crux of the appeal was that allowing the jury to hear details of his client's "bad reputation" led to them making a "knee jerk" and "unfair" decision.
He said hearing the trials together was unfair and prejudicial to Mitchell and it was likely the jury "could not overcome" knowing about his previous offending when deliberating the case.
They may have reached a "knee jerk" verdict after hearing about Mitchell's "bad reputation".
Ryan said joining the two sets of offending in one trial was "illegitimate and unfair".
He also argued there was "stark differences" between the offending at Riverhead and in 1992.
"They should have had different trials," he said.
Ryan also submitted that there had been a "significant" time delay between the three cases the jury heard about at the trial.
There was 25 years between the offending in 1992 and the Riverhead attack.
Further, there was 33 years between the rape of the woman in 1984 and Riverhead.
Ryan argued that the incidents - particularly the Riverhead event where the victim had limited memory of what happened - could not properly be compared.
Crown Law lawyer Charlotte Brooke accepted the gap between the offending was lengthy, but the similarities between the incidents was crucial.
She said Mitchell's mode of offending was "highly unusual".
The panel of judges, who before the hearing had read substantial submissions from both sides, retired for several minutes to discuss the appeal.
They then returned to court and ruled that the appeal was dismissed.
Mitchell's convictions and sentence still stand.
The decision today means a suppression order stopping the Herald publishing details of a rape Mitchell was jailed for as a teenager no longer stands.
He was sentenced to five years in prison for the attack on a woman in her own home in 1973.
The Herald has spent months, and thousands of dollars in legal fees, getting a permanent suppression order preventing us from publishing details of the case, revoked.
READ MORE HERE: Riverhead quarry attacker Colin Mitchell has been raping since 1973 - and now we can finally reveal details
