Makayla McEvoy suffered cuts and bruises when she was pinned to the ground and kicked in her face, arms and back by the animal at a picnic site in Australia.
The schoolgirl was playing near a group of kangaroos when one charged at her.
"I thought I was going to die. All I could see was grey, grey, grey," she told Sky news.
Her mum, Emma McGovern, said: "It just jumped on her back and knocked her to the ground and started. It was just jumping up and jumping up on top of her and scratching her."
The ordeal happened at Wyangala Dam, a popular picnic and camping area about 186 miles (300km) west of Sydney.
When the animal attacked, Makayla's stepdad, Mitch McGovern, ran to save her.
"As I jumped over the top of Makayla to get rid of the kangaroo, all I could see was she was laying, sort of, face down on the ground with her shirt was all ripped, and her back was cut," he said.
"There was no noise, I thought the worst.
As I went to turn back, it actually came back at me, jumped and kicked me in the back of the leg.
Mrs McGovern added: "Honestly, I thought she was dead. She didn't make any noise. She didn't scream for help."
Grey kangaroos can grow as tall as 9ft (2.8 metres), and weigh up to 10st (66kg). The park management said they will put up signs to warn people about kangaroo attacks, which are incredibly rare.