The rapid increase in urban population in Port Harcourt City necessarily came with the massive expansion of the city. Available evidence shows that the newly developed areas of the city are not guided by the national urban development policy and any plan therefore, developments in these areas are unplanned and uncontrolled. Some of these areas are Choba, Omagwa, Eleme, Oyigbo, Aluu and Eneka. This paper is a discourse on the impact of non-adherence to public policy on urban development on environmental sustainability in Port Harcourt City. It identified the most recent urban development policies in the country to be the Nigerian urban and Regional Planning Policy (Act) of 1992 and the National Urban Development Policy 2006.  The work reveals that the nonimplementation of urban development policy in Port Harcourt, especially in the new suburbs of the city has grave implications for environmental sustainability. Some of these consequences are flooding in various parts of the city at certain period of the year, serious traffic-jam and inadequate social amenities in the city. Thus the paper recommends that to achieve and/or enjoy environmental sustainability in Port Harcourt City the relevant State agencies, particularly the Ministry of Urban Development must redouble its efforts to ensure compliance with existing urban development policies and plans in the nation and state, among others.