Fleming attended University College Dublin where he studied for four years. He earned a Bachelor of Science in Experimental Physics. In 1980 he achieved a Master of Science following two years of research in the Atmospherics Group of the UCD Physics Department.[1] When he left university, he moved straight to Irish Met Service. After sixteen months training, he was deployed to Dublin Airport as an aviation forecaster. He moved to the Glasnevin base two years later.[3]
He gave his first weather forecast in July 1985. In 1990, he was made co-ordinator of weather at RTÉ Television.[3] While on television he was noted for his tendency to wink.[4][5]
He has served as Chairperson of the International Association of Broadcast Meteorology and co-chair of the First World Conference on Broadcast Meteorology.[1] He has also chaired the Expert Team on Media Issues for the World Meteorological Organisation and since 2005 has been Chair of the Public Weather Service Delivery Programme Group for WMO, and a member of the Management Group on the WMO Commission for Basic Systems.[3]
Fleming was Head of the General Forecasting Division in Met Éireann until December 2017, when he retired.[1][6][7] In that position he was a primary representative of Met Éireann to the media.[8] He was also a regular contributor to Mooney on RTÉ Radio 1. Now he is involved with World Bank and travels around the world helping countries improve their weather stations, improving safety for the people in those countries. He does this by working alongside them to determine improvements they can make. Some of this work he can do from home. However, he can also be away for months at a time.[9][10] He is the treasurer of the {{usurped|1=[https://web.archive.org/web/19981202073323/http://www.iabm.org/ International Association of Broadcast Meteorology (IABM).