Ngata, of Tongan ancestry, was born in Inglewood, California. He attended Highland High School, where he played on the football team, in Salt Lake City, Utah. He was a three-year starter on the defensive line. As a senior, he recorded over 200 tackles and led his team to the state quarterfinals, following a 12-2 record and a berth in the State Championship as a junior. Ngata was named the 2001 Utah Gatorade Player of the Year and a first-team "USA Today" All-USA selection. He played in the 2002 U.S. Army All-American Bowl.