He worked for the J. Walter Thompson advertising agency before, during, and after his career in Parliament, and was Chairman of the Halifax Building Society from 1983 to 1990.

His studies were interrupted by five years of service as an officer in the King's Royal Rifle Corps in the Second World War.

He spent a year as a marketing trainee with Unilever from 1951 to 1952, and then moved to the J. Walter Thompson advertising agency as a copywriter, before concentrating on a political career.

He fought (and lost) as Conservative candidate for Walthamstow West in the 1955 general election, losing to the incumbent, leader of the Labour Party and former prime minister Clement Attlee.

Hornby was finally elected Member of Parliament at the by-election in June 1956 for the safe Conservative seat of Tonbridge, although, against a local Labour politician and with the unpopular government of Anthony Eden, the Conservative majority was cut to barely 1,600 votes.

Declining the opportunity to stand in the new safe seat of Royal Tunbridge Wells, he returned full-time to J. Walter Thompson, becoming a director.