His career with Plessey was dominated by a battle with Arnold Weinstock of GEC for control of the company together with English Electric from the 1960s.
He then spent a year with American battery and electrical manufacturer P. R. Mallory and Co Inc before joining the Plessey board in 1953.
Clark took over the chairmanship of AT&E and British Ericsson, and in 1964 called in McKinsey's, the management consultancy company, to advise on reorganization.
There was concern that Plessey might be the next target for GEC and an approach was made to English Electric's chairman Lord Nelson.
At Plessey's Caswell Research Centre in Northamptonshire, equipment for digital exchanges, known as System X, was developed.
[2] In 1988 GEC announced a new partnership with the German electronics group Siemens to once again bid for Plessey.