He said he attempted suicide during his NFL career.
He expressed being uncomfortable as "one of just a handful of minorities in elite private schools."
"You learn to tone down your size & blackness by becoming shy, introverted, friendly, so you won't scare the little rich white kids or their parents," he wrote. "Neither black nor white people accept you because they don't understand you. It takes away from your self-confidence, your self-worth, your sanity."
Harvard-Westlake officials said they had been unaware of Martin's feelings.
"Obviously his childhood was a difficult time," said Terry Barnum, Harvard-Westlake's head of athletics. "We supported him and support him now. We are always looking for ways to be more inclusive and more diverse. That is a foundational principle of our school."
Martin, who went on to Stanford, was elected to Harvard-Westlake's sports Hall of Fame in 2014. Because he has been playing in the NFL until his recent retirement, his induction ceremony had not been scheduled.
Barnum said Martin's honesty will help Harvard-Westlake move forward.
"It gives us an opportunity to look inward and look for ways to make our community more inclusive so that kids in the future feel included and welcomed," he said.