The 42 year-old, who operates in the semi-pro ranks, recovered from a bad cut on his right eye to halt his opponent in the fourth.
Speaking after the bout, Froch – brother of world champion Carl watching at ringside – savoured the moment.
“It was a fantastic night for boxing in Nottingham and we did the Motorpoint Arena proud,” Froch told the Nottingham Post. “I would just like to thank the fans who purchased tickets for the event.
“I’ve got nothing but respect for John Lewis. He was strong, he was tough and he hurt me. He tested my heart and I had to dig deep. I found something new in me that I never knew I had.
“I was cut, I was hurt and I just dug in. I looked at my brother, listened to what was said. I was breathing slightly heavy. But they told me to stand my ground and that I would stop him.
“I listened to what they said and got to work. I caught him in the first 15-20 seconds. I caught him again and then I landed a big right and he was KO’ed.
“It was my most enjoyable and memorable win to date,” he added.
Froch is already pencilled in for a first defence of the strap in September after scoring another impressive win in front of his hometown crowd.
Younger brother Carl, who retired in 2014, once fought in front of 80,000 fans at Wembley Stadium.
