"God is great! He moved the mountain! Sarah got THE CALL," wrote Janet Ruddock Murnaghan on her Facebook page.
The Pennsylvania girl, who is critically ill with cystic fibrosis, was entering surgery early Friday, her mother said.
The case drew international attention when the child's family pleaded with the US government to bend the rules and allow her to be put on the list for an adult lung transplant.
It was unclear whether the donated organs, which arrived late Tuesday, came from an adult or a child.
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"Please pray for Sarah's donor, her HERO, who has given her the gift of life. Today their family has experienced a tremendous loss, may God grant them a peace that surpasses understanding," her mother wrote.
"Today is the start of Sarah's new beginning and new life!"