A 71-year-old man has been declared innocent in the US state of Oklahoma after spending nearly 50 years in prison for a murder he did not commit.

Glynn Simmons, who is Black, served more time behind bars before being exonerated than any other inmate in US history, according to The National Registry of Exonerations.

Simmons was released in July after serving a total of 48 years, one month, and 18 days in prison.

Simmons and another man, Don Roberts, were sentenced to death in 1975 for the murder the previous year of a 30-year-old liquor store clerk during a robbery in Edmond, Oklahoma.

Their sentences were later commuted to life in prison.

Simmons and Roberts were convicted solely based on the testimony of a teenage customer who was shot in the head during the robbery but survived.

She picked them out of a police lineup but a subsequent investigation cast significant doubt on the reliability of her identifications.

Both men had also claimed at trial that they were not even in Oklahoma at the time of the murder.

US District Court Judge Amy Palumbo threw out Simmons' conviction in July and declared him innocent at a hearing in Oklahoma County District Court on Tuesday.

"This is a day we've been waiting on for a long, long time," Simmons told reporters. "We can say justice was done today, finally."

Roberts, Simmons's co-defendant, was released from prison in 2008, according to The National Registry of Exonerations.

Simmons may now be eligible for compensation.

"What's been done can't be undone but there could be accountability," he said. "That's what I'm about right now. Accountability."