Mattock was a Leicester-born full-back who was full of promise when he broke on to the first-team scene. He was part of the club's U18 side that won the FA Premier Academy League in the 2006-07 season and was voted Leicester's Academy player of the season.
He made his senior debut in the 2-1 home defeat against Norwich City in April 2007 and he never looked back, catching the eye of a number of scouts from the higher divisions as a result of his top-drawer performances. He signed a new three-year deal with City in November 2007 but, following the club's relegation that year, he handed in a transfer request during the close season amidst reported interest elsewhere.
Mattock remained at the club for the following season, helping City to promotion back to the Championship, but moved on to West Brom in August 2009. His eagerness to leave City didn't go down well with the Leicester faithful, so much so that he was booed on his return to the Walkers Stadium with the Baggies.
Whilst at City, Mattock represented England at U17, U19 and U12 level.