Nicholson already had published his first work in 1926, Birds in England, and had three similar books published soon after. In The Art of Bird-Watching, he discussed the potential of co-operative birdwatching to inform the conservation debate. This led, in 1932, to the foundation of the British Trust for Ornithology, of which he was the first treasurer and later chairman. In 1947–1948, with the then director general of the United Nations' scientific and education organisation UNESCO, Julian Huxley, he was involved in forming the International Union for the Protection of Nature . In 1949 he oversaw Part 3 of The National Parks and Access to the Countryside Act 1949 which established the Nature Conservancy, a British state research council for natural sciences and 'biological service', and allowed for the legal protection of National Nature Reserves and Sites of Special Scientific Interest. He replaced Captain Cyril Diver as Director General of the Nature Conservancy in 1952, serving until 1966, just after the Conservancy lost its independent status. During his leadership the Conservancy established itself as a research and management body which promoted ecology as having broad relevance and application to land use decision-making and management. Monks Wood Experimental Station, which helped set up, was perhaps the first to examine the effect of toxic chemicals on wildlife. In 1952, while in Baluchistan, he contracted polio, which left him with a limp. In 1961 Nicholson, together with Victor Stolan, Sir Peter Scott and Guy Mountfort, formed the organising group that created the World Wildlife Fund . He was also a founder of the International Institute for Environment and Development. In 1966 he set up and headed environmental firm Land Use Consultants, remaining with them until 1989. One of LUC's first reports was 'Parkways in principle and Practice', in which Nicholson urged that "the problems of recreation, traffic, environmental quality and conservation should be studied together..", to form a category of parkways in Britain. From 1951 to 1960 he was the senior editor of "British Birds" and was the chief editor of The Birds of the Western Palearctic from 1965–1992. He was the only author to stay with the project from start to end, personally writing the habitat sections of all species in the nine volumes. In 1976 he was an instrumental part of the setting up of Britain's first urban ecology park and the Trust for Urban Ecology. In 1978 Nicholson was instrumental in founding the ENDS Report which became a highly influential journal for environmental policy specialists. He was President of the Royal Society for the Protection of Birds from 1980–1985, helped set up the New Renaissance Group and was a trustee of Earthwatch Europe. In 1995 he appeared as a guest on Desert Island Discs.
Nicholson married Mary Crawford in 1932 and they had two children, Piers and Tom. The marriage was dissolved in 1964 and Crawford died in 1995. Nicholson then married Marie Mauerhofer in 1965; they had one child, a son, David. She died in 2002. Max Nicholson died in 2003, aged 98.
Legacy
Every year on Nicholson's birthday, 12 July, a group of people walk a section of the Jubilee Walkway in London to celebrate his work in establishing the route. Two memorial sundials have been put in place in memory of Nicholson - one by the Wildfowl & Wetlands Trust at the WWT London Wetland Centre in Barnes, London, and another at Sedbergh School in Cumbria, where Nicholson went to school.