John was born in Waterford in the Republic of Ireland, and graduated with a B.A. (Mod.) in Biochemistry from Trinity College, Dublin in 1993. During this time John became very interested in intracellular signalling transduction, and so in October 1993 he joined the lab of Sir Philip Cohen at the MRC PPU in Dundee. In September 1994, John's first paper reported the discovery of the first member of the p38 MAP kinase, which plays a key role in mediating intracellular responses to cellular stresses and infection by pathogens. After completing his PhD studies, John carried out postdoctoral research with Steve Jackson at the Gurdon Institute in the University of Cambridge, where he discovered a subunit of the yeast ATR protein kinase, that targets ATR to sites of DNA damage in cells.

In 2002, John returned to Dundee to set up his own lab at MRC PPU and became a Professor in 2011. John was a member of the EMBO Young Investigator Programme from 2006 to 2009, received the Colworth medal from the Biochemical Society in 2008, and in 2017 was elected a fellow of the Royal Society of Edinburgh.