The Plessey Company was founded in Marylebone, London, in 1917. In addition to electronic components, the company manufactured detector receivers and telephones for Marconi, and from 1929 Plessey also manufactured equipment for the television pioneer Baird.

After 1936 Plessey manufactured components for the aircraft industry and in the years of the Second World War they produced telecommunications equipment in large quantities, such as the legendary receiver R1155 and the associated transmitter T1154, production was partly transferred to new factories in a tunnel of a projected spur line of the London Underground in Swindon.

In the post-war years Plessey changed production to civilian consumer goods, an early form of an integrated circuit was developed. Together with the British Ericsson Telephone Company and Automatic Telephone and Electric (AT&E), Plessey became one of the UK's most important manufacturers of telephone equipment, and together with Atlas Computers developed a digital telephone system.

Together with GEC, Plessey merged to become Plessey - GEC Telecommunications. After an „unfriendly takeover“, Plessey - GEC was integrated in the Siemens group.

Plessey also repeatedly carried out orders for the benefit of the Army, and from 1959, after the merger with GEC, also orders for the British Air Defence.