Mattock joined Albion for an undisclosed fee in August 2009.
The defender penned a three-year contract, plus an extra year's option in the club's favour.
Mattock's start to last season was hampered by a thigh injury he picked up in training in the first week.
When he did recover he could not force his away into the club's first-team plans due to the form of Nicky Shorey and Marek Cech.
He eventually joined [[Sheffield United F.C.|Sheffield United}} on loan in January until the end of the campaign, making 13 Championship appearances.
He battled with Cech for the left-back spot during his first season at the club, making 29 starts and five appearances in all competitions.
His first-ever goal for the club in February 2010 was a last-gasp leveller in a 2-2 FA Cup fifth-round draw at Reading to take the tie to a replay.
Mattock, who had already been capped five times by England Under-21s when he arrived at The Hawthorns, can also operate at left-midfield.
He made his professional bow for previous club Leicester at the tender age of 16, coming on as a second-half sub against Norwich at the Walkers Stadium in April 2007.
The Leicester-born player, who has also represented England at Under-15, 16, 17 and 19 levels, established himself in the Foxes line-up the following campaign, making 33 appearances.
One of those more memorable games was his impressive performance in City's 4-1 victory at The Hawthorns in March 2008 - the Baggies' final league defeat before lifting the Coca-Cola Championship trophy.
Mattock also played a key role in the Foxes' League One title success during 2008/09, making 36 appearances and scoring his first-ever senior goal in a 2-0 win at Yeovil in January.
In total, Mattock played 73 times for Leicester, including 12 appearances as a sub, scoring once. (LAST UPDATED JULY 13, 2011)