Mortimore captained Chelsea on occasions, the first time in April 1959 in the absence of regular skipper Peter Sillett, and he was also part of the side that represented Chelsea for the first time in European competition, in the Fairs Cup in the 1958/59 season.
Though not the fastest, Mortimore was a tight marker and good tackler. He was strong in the air and had good distribution with his feet. He was nicknamed Percy by the rest of the players and was popular with everybody with whom he came into contact, in his playing days and in the years that followed.
His big reward for his long Chelsea service came in his final season when although less involved due to the emergence in his position of Marvin Hinton, he played the second leg of the 1965 League Cup final away at Leicester, and helped keep a clean sheet in a 0-0 draw that ensured the club won our first domestic trophy following a home-leg win.
There was also a run to the semi-final of the FA Cup where Chelsea were defeated at Villa Park by Liverpool after Mortimore had a headed goal controversially ruled out when it was 0-0.