He read History at the University of Oxford from 1959 to 1963, and then worked in manufacturing (textiles), marketing (detergents), advertising and financial public relations, before taking an MBA at Cranfield University in 1968.

He was the Ecology Party's economics spokesman and press secretary between 1977 and 1980 (the party office at that time being his flat in Hampstead). In 1979 he began studies in economics at Birkbeck College, University of London, completing an MSc in 1983 and a PhD on the economics of the market for positional goods in 1988. He was Honorary Treasurer and then Chairman of the Soil Association between 1984 and 1991. He has been a regular contributor to Country Life magazine, and been published in Prospect and other journals, as well as in academic literature. He was editor of The Countryside in 2097, published in 1997, and gave the third annual Feasta lecture in 2001.

From 1977 to 1995 he worked as an independent consultant in environmental policy and business strategy for the financial services industry. In this time he edited a manual on the formation and management of investment funds in the Former Soviet Union, which was published in 1995.

Since 1995 he has been writing and lecturing on the environmental and social issues which he expects to have a major impact on the global market economy in the 21st century, including oil depletion and climate change.

He founded The Lean Economy Connection to work on the application of Lean Thinking to economic theory and society in general. He is the originator of, and a strong advocate for, the concept of Tradable Energy Quotas (formerly known as Domestic Tradable Quotas ), and an ardent critic of nuclear power.

In his 2007 book The Lean Guide to Nuclear Energy: A Life-Cycle in Trouble, Fleming argues:

• Every stage in the nuclear process, except fission, produces carbon dioxide. As the richest ores are used up, emissions will rise.
• Shortages of uranium - and the lack of realistic alternatives - leading to interruptions in supply, can be expected to start in the middle years of the decade 2010-2019, and to deepen thereafter.
• It is essential that radioactive waste should be made safe and placed in permanent storage. High-level wastes, in their temporary storage facilities, have to be managed and kept cool to prevent fire and leaks which would otherwise contaminate large areas.
• The world's endowment of uranium ore is now so depleted that the nuclear industry will never, from its own resources, be able to generate the energy it needs to clear up its own backlog of waste.

“Large-scale problems do not require large-scale solutions; they require small-scale solutions within a large-scale framework.”

“Localisation stands, at best, at the limits of practical possibility, but it has the decisive argument in its favour that there will be no alternative.”