He then started sending pictures to agents, desperate to earn a living from his obsession with drawing. Later, he scribbled in lay-bys, when he should have been selling tiles and machine tools as a travelling rep. From 1958 to 1963 Gren was employed as an engineering designer.
Gren's first published drawing was a joke cartoon for Spick & Span, and afterwards he sold his first news-related cartoon to the Birmingham Mail, with the help of John Philpin Jones ("Jon") of the News Chronicle: "When I first started I spent a lot of time with Jon, of the Daily Mail. He said draw what you know. And I knew about rugby and the Valleys.".
In 1960 Gren became one of the founder members of the "Knights Of The Round Table", a pop group that would later transform itself into the successful satirical group the The Barron Knights. For a number of years Jones then worked as a freelance cartoonist before in 1968 getting a staff job on the Western Mail. Gren produced a daily topical cartoon, but is best known for creating the weekly strip "Ponty an' Pop" in the Saturday Sports edition of the Western Mail's sister paper South Wales Echo.
With the words of Jon in his mind, the cartoons depicted South Wales valley life, centred on the village of Aberflyarff in Scrumcap Valley on the River Efflew. Rugby, tightly-packed terraced streets, and local politicians all supplemented popular recurring characters, including: Nigel and Neville the message-bearing sheep; Ponty and Pop; and Bromide Lil, the tattooed barmaid of the Golden Dap.
Owning an “original Gren” was an honour and appearing in one an even greater honour, which his friends Max Boyce, Harry Secombe and Wyn Calvin all did. He was also a fast and professional worker: after a morning discussion with the paper's editor, his cartoons were drawn and ready to go by 9.50am - always topical and funny, they were never offensive.
His strip "Big Deal", also known as "Threadneedle", was syndicated for thirty-five years, and he has also produced golf, cricket and rugby sporting calendars, as well as some with a business theme - notably for Bemrose Publishers for 20 years. He was official 'war artist' for the Welsh Rugby Union.
His local popularity gave him other opportunities, including drawing the cover for the Max Boyce album "Doctor's Papers", and resultantly became the first cartoonist to receive a gold disc from the record company EMI.
Through his career he produced over 24 books; while for charity he produced his annual rugby calendar as well as selling numerous original sketches. He was voted best provincial cartoonist in Britain by the CCGB four times: 1983, 1985, 1986, and 1987. He was honoured by the Variety Club for his charity work; given an honorary degree by the University of Glamorgan in 2004; and was made an MBE for services to newspapers in 1989.
Gren retired on his 65th birthday, and said of his job: "When I started out I enjoyed the same things as I'm drawing now. Wales, rugby, local politicians, anything we in this part of the world are able to relate to - I aim to reflect our life and it goes down well with the readers. I'm not trying to prove any points. I try not to get into the political area as that isn't my audience." Gren continued to produce cartoons for the Echo evening paper from home.
Fellow cartoonist Bill Tidy paid tribute to Gren: "I was a great admirer of Gren. To me he was a Max Boyce on paper and a Harry Secombe in humour. He raised a large amount of money for charity, he was crazy on rugby and was the only Welshman I'd ever met who sang out of tune deliberately. He had his finger on the pulse of Wales."
Gren continued to work to the end, with his constant friend and pal at his side: Charlie Friday of Llandaff - a Cocker Spaniel.