County Court Chief Judge Peter Kidd will sentence Pell in a hearing which began at midday.
Pell will also be registered for life as a sex offender.
Chief Judge Peter Kidd told the Melbourne court that the sentencing reflected the crimes committed and were not directed against the Catholic Church.
"Viewed overall, I consider your moral culpability across both episodes to be high", he told the packed court room.
Pope Francis' former finance minister was convicted by a unanimous jury verdict in December of orally raping a 13-year-old choirboy and indecently dealing with the boy and the boy's 13-year-old friend in the late 1990s, months after Pell became archbishop of Melbourne.
Pell - a former top advisor to Pope Francis - is the most senior Catholic to be convicted for child sex offences.
The cardinal, 77, maintains his innocence and is appealing against his convictions.
"In sentencing you today, I am not sentencing the Catholic Church ... it is George Pell to be sentenced".
"I am conscious that the term of imprisonment, which I am about to impose upon you, carries with it a real, as distinct from theoretical, possibility that you may not live to be released from prison", he said. He faced a maximum of 10 years in jail for each charge.
"I think we have to wait and see what happens at the appeal", Mr D'Souza said.
"I am doing my best to hold myself and my family together", said the victim, who can not be identified under Australian law protecting the identity of sex abuse victims.
"I am not sitting in judgment of the Catholic religion or the Catholic Church", Judge Kidd added.
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"Given the speculation and outpouring of anger and distress over the conviction, the reaction to sentencing will be likely highly emotionally charged and extremely polarizing", said Cathy Kezelman, president of the Blue Knot Foundation, a support group for victims of childhood trauma.
"I'm also a survivor and these are for my friends and those victims who haven't survived", she said.
It's expected he will receive a sentence of several years.
The father is considering suing Pell and the church over the abuse.
Pell's first attack on the boys in the priest's sacristy at St Patrick's Cathedral "involved a brazen and forcible sexual attack", Judge Kidd said.
"It is hard for me to allow myself to feel the gravity of this moment, the moment when the sentence is handed down, the moment when justice is done", the man said in a statement read outside court by one of his lawyers.
Judge Kidd rejected Pell's defence argument the crimes were committed by Pell, the man, not the archbishop.
The following article discusses child sexual abuse.
Pell did not take the stand in his defense, but in a video of his police interview in Rome in 2016, he called the allegations against him "deranged falsehood" and "a load of absolute and disgraceful rubbish".
Anticipating huge public interest in the sentence, the judge will permit a camera in court to broadcast his remarks live globally.
Pell abused the 13-year-old choir boys in a Melbourne cathedral in 1996, a jury ruled last year.