David Cook was elected to Belfast City Council in 1973, representing the Alliance Party. He served on the Council until 1985.

Cook was a founding member of the Alliance Party of Northern Ireland. In 1971, he was elected on the Party's Central Executive. He stood in the constituency of South Down in the 1974, 1983 and 1987 Westminster elections and in the 1984 European Parliament election, but was not elected. Cook was successfully elected to the Northern Ireland Assembly in October 1982.

Cook was the first non-Unionist Party member to serve as Lord Mayor of Belfast since partition (Sir Edward Coey, a member of the Liberal Party, served as Mayor in 1861. Viscount James Pirrie, a prominent Liberal Unionist, served as Lord Mayor in 1896 and 1897).