The sentence was announced by an Anti-Terrorism Court (ATC) in a trial held inside a prison on Monday.
Imran Ali, a resident of Kasur city, was involved in at least nine incidents of rape-cum-murder of minors, including Zainab, which he had confessed to during her murder investigation, Dawn online reported on Tuesday.
Ali was also ordered to pay a massive fine. His failure to pay the fine will result in an additional six months of imprisonment, the court said.
On Saturday, he was given death sentence on 12 counts for the rape and murder of three other minor girls. Three more cases remain pending against Ali.
Zainab's rape and murder had sparked outrage and protests across the country. The girl went missing on January 4 and was found dead in a trash heap in Kasur on January 9.
The government had declared Ali's arrest on January 23. He was given death sentence for Zainab's rape and murder in February.
He challenged it but the apex court rejected his appeal on June 12, saying that the petitioner had admitted committing similar offences with eight other minor victims.
Ali still has the right to seek clemency from President Mamnoon Hussain.