David Somerville Cook, English-born solicitor and politician, is born on January 25, 1944. He is a founding member of the nonsectarian, liberal-centre Alliance Party of Northern Ireland (APNI). He serves on Belfast City Council from 1973 to 1986, and in 1978 he becomes the first non-Unionist Lord Mayor of Belfast since 1898. He is elected as a member of the Northern Ireland Assembly of 1982 and serves on that body until its abolishment in 1986. He is appointed Chair of the Police Authority of Northern Ireland in 1994 and holds that position until his resignation from the role in 1996.

Cook is born to Francis John Granville Cook and Jocelyn McKay (née Stewart) in Leicester, England. As a child, he moves to Northern Ireland with his parents and sisters after his father is appointed headmaster of Campbell College in 1954.

Cook works as a solicitor, eventually becoming a senior partner at Sheldon and Stewart Solicitors.

In 1970, Cook is a founder member of the Alliance Party of Northern Ireland, a nonsectarian party, while he is elected to the party’s Central Executive in 1971.

Cook is elected to Belfast City Council in 1973, a position he holds until 1985. In 1978, he becomes the first non-Unionist Lord Mayor of Belfast since William James Pirrie, a Home Rule Liberal, in 1896–1898.

Cook stands for APNI in Belfast South in the February 1974 United Kingdom general election, taking just under 10% of the vote. He is able to improve to 27% of the vote at the 1982 Belfast South by-election. Following this, he wins a seat on the Northern Ireland Assembly representing Belfast South.

In the 1983 United Kingdom general election, 1986 Belfast South by-election and 1987 United Kingdom general election, Cook consistently wins over 20% of the votes cast in Belfast South. He also stands for Alliance in the 1984 European Parliament election but takes only 4% of the vote. From 1980 to 1984, he serves as the Deputy Leader of APNI.

In 1994, Cook becomes the Chairman of the Police Authority of Northern Ireland, but he is sacked from this role in 1996 after losing a vote of confidence. After a critical account of his role in an internal row in that authority appears in newspapers in 1998, he undertakes a lengthy libel case which is ultimately settled out of court. He subsequently sits on the Craigavon Health and Social Services Trust.

On September 20, 2020, it is announced that Cook has died after being diagnosed with COVID-19 during the pandemic. According to his family, he dies on September 19, 2020, at Craigavon Area Hospital. He had had a stroke two years earlier. He is survived by his wife Fionnuala, his sisters Alison and Nora, his daughter Barbary, his sons John, Patrick, Julius, and Dominic, and his granddaughters Romy and Imogen.