Mr Mullen has since received five Osman warnings, which mean police have intelligence about a threat to life but not enough to justify arresting the potential offender.
“The police have offered me witness protection after each death threat but each time I’ve turned it down because I want the option to go back home,” he said.
“They would have made me start again, changed my name,” he added.
Mr Mullen, who was once a senior member of National Action, said the most recent “credible death threat” had come in February of this year.
However, he said he would chose to reveal the plot again, even with the consequences to his life.
“Doing what I did to foil a murder plot has turned my life upside down. It’s scary. But I wouldn’t do anything different if I could do it all again," he told The Independent.
"National Action had to be stopped. It’s as simple as that."
Renshaw has admitted to making preparations to kill Ms Cooper – including purchasing a 19-inch Gladius knife – and will be sentenced next month.
However, he denied that he was a member of National Action, which was banned in December 2016 over its support of the murder of Labour MP Jo Cox.
Earlier this month, a jury failed to reach a verdict on whether Renshaw was a member of the group.
National Action was the first extreme right-wing group to be outlawed by the government since the Second World War.
Matthew Collins, Hope Not Hate's head of intelligence, warned that members of National Action are "dangerous and incredibly extreme".
"We knew, and Robbie knew, that they wouldn’t react kindly to their exposure. Violent threats from violent extremists is to expected, and we have taken every precaution to keep Robbie safe," he said.
"‘Osman’ warnings are given on the back of credible police intelligence. We therefore look forward to the police making arrests.”
In July 2017, Renshaw was on police bail for making hate speeches and under investigation for child sex offences.
He had planned to take revenge by killing Ms Cooper and a female officer, DC Victoria Henderson, who was involved in the investigation.
Renshaw told Mr Mullen and other National Action members about the plan at the Friar Penketh pub in Warrington.
“Renshaw told his plot over two or three hours. It wasn’t just a quick outline. He kept on explaining, saying he’s got the machete, that it’s designed for cutting through pigs. And the pig is the closest to human flesh,” Mr Mullen told The Observer.
“There was an aura around the room. It was as if this is what they had been waiting for; everybody sort of had a smile on their face.”
After leaving the pub, Mr Mullen contacted Mr Collins to warn him of the plot.
Agencies contributed to this report