Lewis was born in Stornoway in the Isle of Lewis on New Year's Day, 1957. He left the island just before his eleventh birthday when his family moved to Insch in Aberdeenshire, and his secondary education was at Inverurie Academy. He moved to Aberdeen to attend University in 1974, and the city has been his adopted home for most of the time since then.

Lewis is married to Sandra, they have two children, one at primary and one at secondary school, and lots of family members also live in and around the city. Lewis has represented the Central constituency since the Scottish Parliament was set up in 1999.

Lewis served as a Deputy Minister in the Scottish Executive from 2001-2007. His responsibilities have included transport, planning, enterprise, energy, lifelong learning, environment and rural development health and community care. Lewis is now the Shadow Minister for Energy, Enterprise, is a member of the Health and Sport committee and is co-convener of two Cross Party Groups; Food and Oil and gas.

Lewis is a member of the trade union Amicus, the Scottish Co-op, Grampian Racial Equality Council and Aberdeen Council of Voluntary Organisations. He is a patron of the AFC Supporters' Trust.