Williams, who is a Welsh Premier League board member, joins 17 other club representatives including legendary former players Luis Figo of Inter Milan and Emilio Butragueno of Real Madrid.
England are represented by Arsenal and Tottenham Hotspur, who join other major European clubs which include Villareal of Spain and Borussia Dortmund of Germany.
The ECA is the sole independent body representing football clubs at European level and replaces the G14 Group - which famously proposed a breakaway move by major clubs - and the European Club forum.
It is fully recognised by UEFA and FIFA to work towards building the future of European football.
The competitions working group leads the managment and control of the club competitions through the relevant UEFA and FIFA club football committees.