The bricklayer became the ITV1 gameshow's first millionaire on Saturday. He told The Sun in an interview that he was "glad" of his triumph.
Hageman said: "This win hasn't sunk in - it was just hours ago that Simon Cowell had his arm around me and was telling me, 'Well done'.
"But I'm glad it's happened. I want to turn my life around. Three years ago, I was in jail."
Hageman revealed that he was sentenced to a five-year prison term for breaking into a man's house and attacking him following an argument over a woman.
He explained: "I got mixed up with the wrong woman, someone I shouldn't have been involved with. I was doing things I regret, drinking and dabbling in drugs.
> Simon Cowell's 'Red or Black?' crowns second millionaire Kevin Cartwright
"I started getting calls from a guy in Reading. He was very threatening... I tried to brush it off but he kept saying he was going to harm me."
Hageman said in hindsight that he was an "idiot" for committing the assault, adding: "I was charged with aggravated burglary and ABH. I pleaded guilty straight away as I knew what I'd done was wrong."
The 31-year-old served two and a half years at an Oxfordshire prison and was released in 2009. He served the remainder of his term on licence, and added that he has now made peace with the man he attacked.
Hageman said that he intends to use the money to help support his family, including girlfriend Jane Makepeace and his 62-year-old mother Joyce Sykes.
Cowell said that he believes the money will "make a big difference" to Hageman and his family.
A spokesperson for ITV said in a statement: "Nathan was eligible to take part under the terms and conditions. Background checks were conducted on all contestants.
"We were aware Nathan has served a two-and-a-half-year sentence in prison."
> Simon Cowell gameshow 'Red or Black?' launches to 6.9 million
Move through a gallery of images from the filming of Red or Black? below: