Leach has served as an officer and committee member of numerous New Zealand and international organisations concerned with archaeology and cultural heritage management, and has held honorary fellowships in various institutions.

Much of his life as a young adult was spent as a bushman: possum trapping, deer stalking, scrub-cutting, and working in shearing gangs.

[4] Leach began his teaching career as an undergraduate tutor in the Anthropology Department at the University of Otago from 1967 and joined the academic staff as a junior lecturer in 1969.

[citation needed] During his 20 years at Otago he taught courses on New Zealand and Pacific prehistory, the origins of civilisation, and archaeological methods, and also ran laboratory classes and field schools.

[citation needed] This greatly expanded the opportunities for senior archaeology students to do MA thesis research based on more than visits to the library and minor projects of fieldwork.

[18] Leach has had a strong commitment to "area excavation", in the belief that the reconstruction of prehistory is best approached by first understanding the patterns of human culture in the synchronic dimension before turning attention to diachronic studies.

He participated in two archaeological surveys with Jim Specht for the Australian Museum, on Norfolk Island in 1976,[29] and in the Kandrian district of Southwest New Britain, Papua New Guinea in 1979.

[30] In 1977–1978 he carried out archaeological research on a Polynesian outlier in the Outer Eastern Solomon Islands with Janet Davidson, with a thorough survey and two major excavations in Taumako.

With the full agreement of the Taumako people, the human remains were taken to New Zealand for specialist study and much has been learned about individual life histories, diet, health and disease.

He has always approached the study of prehistoric communities from a strongly scientific perspective, which led him to explore methods for the identification of faunal remains and the sourcing and dating of lithic materials.

[44] The final stage of Leach's professional career was spent at The Museum of New Zealand Te Papa Tongarewa after he retired from university teaching.

In 1988, the director John Yaldwyn encouraged Leach to join the museum as an honorary curator and establish a specialised laboratory for archaeozoology.

This was a bold move at a time when archaeologists struggled to find places for long term storage of excavated faunal material, which could be used for advanced research in later years.

Although the long term research potential of archaeofauna was widely recognised, museums were extremely reluctant to accept large collections of bones, shell and soil residues.

[citation needed] During this period at Te Papa he also ran a consultancy and undertook investigations into aspects of Waitangi Tribunal claims for the Crown Law Office and Maori iwi organisations.

[citation needed] In 1994 Leach was invested as a kaumātua of the Kohunui Marae, Pirinoa (Ngāti Kahungunu ki Wairarapa), a rare honour for a European.