His part in the discovery of Caledonian  Jade Green and subsequent work on blue dyes of the same series placed him among the foremost discoverers of the industry.

John Thomas was born in Whitford, Flintshire and moved with his family to Harlech when he was three years old. His father is described as a Blacksmith/Ironworker. They lived initially in Llechwedd before moving to 6 Tryfar Terrace. 

John left school at the age of twelve to work for Mr Baldwin Lloyd of Bron y Graig at his mineral water factory. His potential was recognised by both Mr Lloyd and his Headteacher Daniel Angell Jones and between them they found funds to sponsor him to Barmouth Grammar School.

In 1904 he won a Sir Alfred Jones scholarship to Aberystwyth University where he obtained a first-class honours degree in chemistry followed by a year of research under Professor Sudborough for which he was awarded an MSc. In 1908 he won a '1851 exhibition' scholarship to Trinity College Cambridge and to further research under Professor Sir William Pope. He left Cambridge in 1911 to work in the aeronautical section of the National Physical Laboratory. In 1912 he moved to Nobels Explosives Company at Ardeer, Scotland. His research work here earned him a D.Sc. from Aberystwyth University. 

In 1918 he moved to be the Chief Chemist and later Managing Director at Solway Dyes Ltd, Carlisle (later Scottish Dyes Ltd) and oversaw a move to Grangemouth. The company specialised in the manufacture of fast dyes of the indanthrone group, these important essentials of the textile industry had not previously been made on a technical scale in Britain, relying instead on imports from Germany. The manufacture of the fast dyes involved the translation of a delicate laboratory technique into large scale practice and for this work John Thomas was said to be gifted with marked originality and breadth of view. In addition to a great scientific acumen and brilliant intuition he also possessed a good business ability. He made outstanding contributions to the development of the British dyestuffs industry. His part in the discovery of Caledonian  Jade Green and subsequent work on blue dyes of the same series placed him among the foremost discoverers of the industry. 160 patent specifications bearing his name testify to the fertility and ingenuity of his mind and demonstrate the value of his contribution to scientific research. 

In 1926 Scottish Dyes Ltd merged with Dyestuffs ICI and John Thomas became Joint Managing Director of the Dyestuffs Group a position he retained until his untimely death in 1933 aged 47 years.

John Thomas is described as an ardently patriotic Welshman, speaking with a broad Welsh accent and including 'English' in any space assigned to 'Foreign Languages'. He was well liked by colleagues who described his boundless energy and burning enthusiasm which was infectious and inspired them in overcoming the innumerable obstacles between a brilliant laboratory discovery and successful manufacture. His retentive memory and mental agility were described as phenomenal, and no difficulties daunted him in pursuit of a scheme once adopted.  At the same time, he is described as having a charming personality which won him a wide and deserved popularity.  He visited Harlech regularly and was a keen golfer particularly enjoying the Royal St David’s course in Harlech where he had once been a caddy.

In 1915 John Thomas married Olive Morgan and they had a son and two daughters. He is buried in Moriah Chapel graveyard in Harlech.