Mae gan John brofiad 40 mlynedd o weithio mewn sefydliadau mawr a chymhleth sy'n gysylltiedig â'r GIG a'r llywodraeth. Bu'n gwasanaethu fel Cyfarwyddwr Gweithredol, ac am gyfnod o chwe mis bu'n Brif Weithredwr dros dro yn Ymddiriedolaeth GIG Gogledd Bryste. Gadawodd y GIG yn 2001 i ymgymryd â swydd Cyfarwyddwr Perfformiad a Gweithrediadau yn Adran Iechyd Llywodraeth Cymru, cyn ymddeol yn 2007. Bu'n gwasanaethu fel aelod annibynnol ar Fwrdd Iechyd Cwm Taf rhwng 2009 a 2017, lle bu ganddo gyfrifoldebau penodol dros faterion ariannol, a bu'n gwasanaethu fel cadeirydd yr is-bwyllgor Archwilio a chadeirydd y pwyllgor ar gyfer Cyllid, Perfformiad a'r Gweithlu.
John has 40 years experience in large and complex organisations with the NHS and Government. He served as an Executive Director, and for a period of six-months as Acting Chief Executive of North Bristol NHS Trust. He left the NHS in 2001 to take up a post of Director of Performance and Operations within Health Department of Welsh Government before retiring in 2007. He served as an independent member of Cwm Taf Health Board from 2009 until 2017 where his particular responsibilities were financial matters and he served as chair of the Audit sub-committee and chair of the Finance, Performance and workforce committee.