Leo was born in Petone, Wellington. The only son of Rawinia Queenie Agnes Bramley and Francis Leo Buchanan he died in Wellington of carcinoma of the bowel.

He married High School teacher Mary English in 1966 in Dipton Southland and they have eight children: Rachel, Felicity, Hannah, Benjamin, Mathew, Emma, Ruth and Joseph, and two foster children, Kathryn and Frances Riordin. He was also immensely proud of his nine mokopuna: Lily, Antonietta, Frances, Henry, Clare, Tusiata, Mathias, Eleanor and Uma.

While the Buchanan side of the family were early settlers from Scotland, his mother's family were associated with the Te Aro Pa in Wellington after his ancestors, Hemi Parai and Tawhirikura, had moved south from Taranaki during the early 19th century to escape tribal warfare. As a result he was proud of his Te Ati Awa, Taranaki and Ngati Ruanui whakapapa and in his retirement came to live on his ancestral lands. Mary's younger brother, William, was a New Zealand Member of Parliament, Minister of Finance and Prime Minister. On Leo's Maori side he was related to the great Taranaki poet J.C. (Jackie) Sturm.

Leo was raised in Karori and educated initially at St Joseph's College Masterton before going to Auckland's Sacred Heart College. He then went on to study medicine at the University of Otago Dunedin. His early hospital years were spent in Timaru (1966), Whanganui (1967) and Invercargill (1998). His postgraduate training started in Melbourne at the Royal Women's Hospital (1969) and the next year at the Queen Victoria Women's Hospital. On sitting his RACP examinations in 1970, he achieved the highest marks for Australasia.

After a further year in Sydney at the Grosvenor Diagnostic Centre he was appointed as the foundation Paediatrician to the Taranaki Base Hospital in New Plymouth. This required the development of all services including a neonatal unit and working to get paediatrics as a specialty recognised by his colleagues as a worthwhile venture. Initially being a sole practitioner meant long hours and little family time. In 1983 he moved to Masterton Hospital where he had the dual role of Paediatrician and Medical Superintendent. Ever since his Melbourne days when he complained to the commission after he and one other registrar were required to toss a coin as to who would get which salary, he had not stepped away from controversy. This did not help his appointment the next year. As a devout Catholic, he was not prepared to sign the abortion papers for the Masterton Hospital which led to difficulties and in 1987 he transferred to Waikato Hospital Hamilton. He became their first Community Paediatrician, providing a service to the wider region with clinics in Tokorua, Taumaranui and Huntly as well as developing all the out-of-hospital requirements in the city.

In 1992 he changed again, back to his turangawaewae in the Wellington region with an appointment to the Hutt Hospital. This was a general paediatric position but he also made good use of his skills in neurology and behavioural paediatrics. He had a special interest in these, in particular attention deficit and autistic disorders, and was a staunch protagonist for breast feeding and Maori health. He also continued his interest in community Paediatrics with a clinic in Upper Hutt and as a part-timer he had a busy private practice.

One of his greatest attributes and loves was teaching and mentoring younger colleagues. The presence of both medical students as well as the junior medical staff gave him ample opportunity for this. The numerous tributes on the Paediatric Society email list server illustrate just how much his teaching and advice were valued by many colleagues over the whole country. He had a prodigious memory and an ability to come up with rare possibilities when his colleagues sought help with clinical dilemmas and could usually provide chapter and verse in references. This was a source of advice and help that was always willingly available.

In spite of having a grandmother fluent in Maori, Leo did not have the chance to learn the language as a child and as an adult had to struggle for every scrap of Maoritanga that he achieved, including a dogged mastery of ceremonial Maori. As a result he was able to lead us at Paediatric Society and College meetings in the formal welcomings. His love of language also ran to his everyday work with lengthy and entertaining letters about his patients that his colleagues still comment on.

He was the initiator and founding chairman of the Maori Health Committee which was set up as a subcommittee of the New Zealand Committee in 2008. He chaired this until 2014. In this position he was a member of the Paediatric and Child Health Committee in New Zealand and as such was the representative at the Child Health Divisional meetings in Australia, contributing his expertise more widely. He was always a staunch advocate for Maori health within the College and initiated a number of training seminars to encourage colleges in their understanding of Maoritanga.

He was appointed to Ministry of Health Child Review of Child Health Services in New Zealand in 1979 and 1999. During his time as medical superintendent in chief of Masterton Hospital he was secretary to the Medical Superintendent's Association of New Zealand from 1983 to 1987. He was a member of the International Advisory Board of La Leche League International from 1980 to 2005. He was a member of the Society for the Protection of the Unborn Child (SPUC).

Leo had not written much in formal medical journals but was more committed to the oral tradition of passing on his wisdom about breast feeding and Maori health in particular to numerous groups over many years.

He was a driving force behind the Maori Trust that built low-cost apartments at Greta Point for descendants of Te Aro Pa. His love of walking helped beat the stress of work, as did his enthusiasm for literature, theatre and music.

He was committed to his church activities and very much a part of the congregation. Accordingly, in the Maori tradition, the family was granted permission to sleep next his open coffin the night before his mass, which was bilingual (Maori and English) and attended by seven priests. He was the first to be buried in the newly blessed urupa, Opau at Makara.