After working as a teacher and gaining a BA from Victoria University of Wellington, he was appointed as a researcher to the Commission on Education.
He performed background research on Treaty of Waitangi claims, and wrote an extensive review for the University of Waikato on their handling of complaints against a doctoral student.
Deakin University recognised Renwick's contributions to Australian education with an honorary LittD degree.
[2] His brothers were a welder and a truck driver, and Renwick's desire at age 14 was to become a carpenter, until his teachers encouraged him to stay on at school.
[2] Returning to Victoria, his 1962 master's thesis was titled Self-government and protection: a study of Stephen's two cardinal points of policy in their bearing upon constitutional development in New Zealand in the years 1837–1867.
Under the chairmanship of George Currie, he was research officer for the Commission on Education from 1960 to 1962, appointed by Keith Sheen, who had already picked him as a potential future Director-General.
"[3] In May 1971 Renwick succeeded Joseph Langmuir Hunter (1908–1985) as Assistant Director-General of Education, under new Director Robin Williams.
His "backwoods ... Depression-era" childhood led to him being described as a "man of the people" who had attained his achievements through hard work and the opportunities afforded him by New Zealand's education system.
[11] The following year Renwick led the New Zealand delegation to the Commonwealth Secretariat in London, where it was reported that Third World countries were finding imported educational systems problematic and might need a move towards more vocational training.
[13][14] Lyall Perris, former acting secretary for education, later wrote "The Report of the Taskforce (Picot 1988) was released in May 1988, a few months after the Lange government had been returned with a sizable majority.
[2] In April 2000, the University of Waikato found itself in the national news when complaints, which had been ongoing since 1995, that a doctoral student researching the German language in New Zealand was a neo-Nazi suddenly became public.
[16] In December 2000, Vice-Chancellor Bryan Gould called on Renwick to provide "a full review of the University's handling of the case",[17] a process that was initially intended to take three weeks.
When a civil servant, he was known to request that his overseas trips be arranged to suit the Phoenix Choir rehearsal schedule.
[22] In 1978, he received a Distinguished Visitor Award from Monbu-shō, the Ministry of Education, Science, Sports and Culture in Japan.
[23] Also in 1988, Deakin University recognised Renwick's contributions to Australian education with an honorary Doctor of Letters degree and he received the Mackie Medal from the Australia and New Zealand Association for the Advancement of Science.
[2] In 2011 Renwick published a biography of Colin Scrimgeour, Scrim: The Man With a Mike, billed as "a cautionary tale of one of New Zealand's most popular radio broadcasters".