On graduation in 1942, Geoff Tootill was directed into Operational Research in connection with Fighter and Bomber Commands. It was 1943 before he managed to get transferred to TRE in Malvern, to work on radiolocation equipment installed in night fighters. FC Williams and Tom Kilburn were already there, developing novel electronic techniques for use by the rest of the Establishment. These three boffins later moved to Manchester University and applied their skills to making the world's first wholly electronic stored program computer, demonstrated in June 1948. Geoff then spent four months on the staff of Ferranti Ltd, designing the first mainframe to be put on sale.

In 1949 he moved to the Military College of Science, where he initiated lectures and lab study on digital computing. During his subsequent service in the Royal Aircraft Establishment he wrote with Stuart Hollingdale "Electronic Computers", Penguin 1965. This paperback ran through eight printings and was translated into Spanish and Japanese. In the International Federation for Information Processing Geoff was Chairman of the drafting committee of its "Vocabulary of Information Processing", North Holland Publishing Company 1966, on which various national and international standards were based.

From 1963 to 69 he worked in France, Holland and Germany planning, setting up and directing the Control Centre of the European Space Research Organisation, including its four ground stations. These were located in Belgium, Spitsbergen, Alaska and the Falklands. He then spent four years as a London civil servant until in 73 he escaped to the National Physical Laboratory, among the executive staff of the European Informatics Network.

Since his retirement in 1982 he has written a few programs, including one for the phonetic transcription of British surnames, which has been a commercial success.