William Powell is active within the third sector in Powys, having served as a founder Trustee of Felin Talgarth Mill (2010 - 2016); as well as being a supporter of Talgarth Tourist Information and Resource Centre (2011 – present); a committee member of Talgarth Festival of the Black Mountains, advisor to Talgarth Luncheon Club, which is currently re-forming post Covid, and a founder member of Talgarth and District Regeneration Group. Since 2021, William has been regional champion of the DPJ Foundation, rural mental health charity and became President of Talgarth Town FC Juniors in summer 2022. William served as a member of Talgarth Town Council from 2008 to 2022 and was elected Chair from May 2018 until October 2020.

He is a member of Powys County Council (2004 – present) and was a member of Brecon Beacons National Park Authority from 2004 – 2011, serving as Chair of BBNPA Audit and Scrutiny from 2009 - 2010 and Chair of the International Dark Skies Board from 2015 – 2020. He was re-appointed to the BBNPA in May 2022 and elected Chair of Audit and Risk Committee in July. In addition, William served as a Trustee of the Bevan Foundation from 2018 – 2021.

William was Assembly Member for Mid and West Wales from 2011 – 2016 and became a member of the Welsh Government EU Advisory Group in September 2016 until its dissolution in 2021. William served as President of the Welsh Liberal Democrats 2017 to 2020 and was recently elected to the Federal Council of the Liberal Democrats.