Educated at the University of Edinburgh and University of Cambridge, Shepherd-Barron went on to work for De La Rue Instruments in the 1960s and came up with the concept of a self-service machine which would dispense paper currency with 24/7 availability. This was the Automated Teller Machine (ATM). The first machine was established outside an Enfield, north London, branch of Barclays Bank in 1967, when he was Managing Director of De La Rue Instruments: there are now more than a million installed worldwide. He received the Order of the British Empire in the 2005 New Year's Honours list for services to banking as "inventor of the automatic cash dispenser".
There is still some controversy over the invention. The first ever ATM was his creation, however a mechanical dispenser had been developed by Luther George Simjian and was installed in 1939 at the City Bank of New York. It was removed from the bank the same year it was installed due to lack of customer demand. Shepherd-Barron's machine was the first true ATM (the idea for which he had in the bath) and was installed at Barclays Bank in North London on 27 June 1967.
The Shepherd-Barron dispenser actually predated the introduction of the plastic card with its magnetic strip: the machines used special cheques which had been impregnated with a radioactive compound of carbon-14, which was detected and matched against the personal identification number (PIN) entered on a keypad. A proposed PIN length of 6 digits was rejected and 4 digits chosen instead, because it was the longest string of numbers that his wife could remember.
As well as the ATM, he has also invented some less successful devices, such as one that plays the sound of a killer whale to deter seals from salmon farms.