In this paper we try to make clear that the original utilitarian economic thought of J. S. Mill, which was like a headwater, ran to Cambridge University, after which his followers could tackle environmental problems of their days from an economic point of view (1). First of all we refer to the utilitarian background of Mill’s theoretical suggestions in his Political Economy and his political activities in the Commons Preservation Society (CPS) and the Land Tenure Reform Association (LTRA) (section 2). Next we introduce two of Mill’s disciples of Cambridge insiders, the economist Fawcett and the moral philosopher Sidgwick, and discuss their theoretical and practical succession to Mill’s thought (section 3). Likewise two Cambridge outsiders, the critic Ruskin and the economic theorist Jevons, criticized Mill’s orthodoxy and influenced new Cambridge insiders. We describe these two outsiders (section 4) and identify one insider, Marshall, who established the foundation of today’s Environmental Economics (section 5). Then we mention his disciple, another insider, Pigou, who developed this study (section 6). Lastly, we discuss methodological criticism of Cambridge Environmental Economic Thought (CEET) and suggest other streams to establish and develop Environmental Economics (section 7).