Lewis studied civil engineering for a year before turning to music. His mother was a shop assistant, and his father worked in management for a chemical company. Lewis attended Hawthorn High School in Pontypridd, where he was in the same year as former bandmate Ian Watkins. His favourite subjects were science and history. His first concert, as a concertgoer, came when he saw Tesla at St David's Hall with Watkins.
Before Lostprophets formed, Lewis and Ian Watkins were bandmates in a band called Public Disturbance. He was initially the original bassist for Lostprophets, but soon became the rhythm guitarist, with Stuart Richardson replacing him on the bass guitar. After the split of Lostprophets in 2013, Mike Lewis became the manager to a few smaller bands. On 1 May 2014 it was announced that the former members of Lostprophets (including Lewis) has formed a new band and are writing new music. On 1 July 2014, the new band No Devotion released their first single, 'Stay' and will release their debut Album 'Permanence' in September 2015.
In September 2006, Lewis married his girlfriend, Amber Payne. They now reside in Malibu a few streets away from bandmate Jamie Oliver. On 24 February 2009, he confirmed on his Facebook page that he and his wife were expecting their first child.
Lewis is an avid Star Trek fan and owns a Star Trek uniform. He is a lifelong Everton fan. He surfs Santa Monica beach almost every day when he is at home. It was previously reported that Lewis was once a vegan who turned vegetarian, but in an interview with The Rock Radio Station in New Zealand, it was revealed that was not the case. He stated to the hosts that he enjoys his steak cooked "medium rare".