Holliday joined Exxon in 1978, where he worked for 19 years until 1997, gaining experience in all aspects of the oil and gas industry. Holliday was made operations manager of the Fawley Refinery near Southampton in the UK when he was 30 years old. In 1998, when British Borneo merged with Hardy Oil and Gas, Holliday became the international director. Holliday joined the National Grid Group as the board director responsible for the UK and Europe, in March 2001. Following the merger of National Grid Group plc and Lattice Group plc in October 2002, he took responsibility for the Group's electricity and gas transmission businesses. In 2003, Holliday was promoted from group director to group director responsible for the UK Gas Distribution and Business Services. Holliday was also appointed the chief executive of Transco. Holliday became chief executive officer of National Grid plc in January 2007. In 2009, Holliday's total compensation for the role of CEO was £2.2 million, consisting of a £929,000 annual salary, and a £1,277,000 bonus. In 2013, Holliday was awarded an honorary degree from the University of Strathclyde for his contributions to the power and energy sector. Holliday viewed the concept of baseload as "outdated", as microgrids would become the primary means of production, and large powerplants relegated to supply the remainder. In November 2015, National Grid announced that Holliday would step down as CEO in March 2016, and that John Pettigrew, the UK executive director who joined National Grid 25 years ago, would succeed Holliday. Holliday left National Grid in July 2016. In March 2017, Greg Clark MP, the UK Secretary of State for Energy, established an independent Inquiry into the conduct of a 2012 decommissioning services procurement process undertaken by the Nuclear Decommissioning Authority and the reasons why the contract subsequently awarded to Cavendish Fluor Partnership in 2014 "proved unsustainable". Holliday was asked to lead the Inquiry. Clark asked the Inquiry to take a ‘cradle to grave’ approach beginning with the NDA’s procurement and ending with the contract termination, to review the conduct of the NDA and of government departments and to "make any recommendations it sees fit". Interim findings were published in October 2017.
In 2009, Holliday achieved an entry in Who's Who, as follows: "Chief Executive National Grid since 2007, b. Exeter 26th October 1956, son of Michael and Jean Holliday m. Kate Patterson; three d. Educ: University of Nottingham. Esso/Exxon, 1978–1997; Ops. Manager, Fawley Refinery, Esso UK, 1988–92; Supply & Transportation Divl Dir, Esso UK 1992–94; Regl Vice-President, Gas, Exxon Co. International 1994–97; Bd Dir, British Borneo Oil & Gas, 1997–2000; National Grid Bd Dir responsible for Transmission, 2001–03; Gp Dir responsible for UK Gas Distribution and Business Services, 2003–07; Non-exec Dir Marks & Spencer, 2004-->; Recreations: sports, Rugby, skiing, arts."
