William Bell was born in Belfast on 9 October 1935, to Robert and Mary Ann Bell. He was educated at Fane Street Primary School and Grosvenor High School. He worked in sales promotion, marketing and public relations. He is a member of the Ulster Unionist Party.

Bell served as a councillor on Belfast City Council from 1976 to 1985 and was the Unionist spokesman on housing from 1976-79. He had previously been elected as a member of the Northern Ireland Constitutional Convention of 1975-6.

From 1982 to 1984, Bell was a member of the Northern Ireland Assembly, and during this period was the Deputy Chairman of the Assembly's Finance and Personnel Committee. Bell was re-elected to the Assembly in 1998 and again in 2003. He chaired the Northern Ireland Public Accounts Committee from 1999 to 2002.

Bell has served on the board of many charitable, educational and voluntary organisation. He sat on the Belfast Board of Harbour Commissioners and was President of Action Cancer.