Born in Karlsbad, Czechoslovakia, Fleischmann and his family fled to Britain in 1939. He was educated at Imperial College, London where he gained his PhD in 1951. He taught chemistry at the University of Durham (1952–57) and at Newcastle University (1957–67). In 1967 he was appointed professor of electrochemistry at the University of Southampton.

By 1988 Fleischmann had an international reputation as a productive and innovative electrochemist. Between 1985 and 1988 he coauthored 29 papers with his former student, the American chemist Stanley Pons (1943– ). Quite unexpectedly, in March 1989, they announced that they had achieved nuclear fusion by an electrolytic method under laboratory conditions at room temperature.

Nuclear fusion, the fusion of two light atomic nuclei to produce a heavier nucleus, is a process accompanied by large amounts of released energy. For fusion to occur the nuclei have to be brought close together, and this involves overcoming a high energy barrier caused by the mutual repulsion of the nuclei.

In practical cases this is achieved by high temperatures, as in the Sun or in thermonuclear weapons. Experimental thermonuclear reactors such as the tokomak also use high temperatures (about 300 million degrees) to initiate fusion. There is, however, considerable interest in methods of initiating fusion at low temperatures – so-called ‘cold fusion’. One approach to this has been inertial confinement, in which a sample of material is compressed by intense laser beams or particle beams.

Fleischmann and Pons also thought that pressure might be a way of initiating cold fusion. Palladium metal has a high affinity for hydrogen and, under the right conditions, can absorb large quantities of it. They electrolyzed water containing the deuterium isotope using a palladium cathode and reasoned that the palladium might absorb so much deuterium that the effective deuterium pressure within the electrode would be high enough to cause nuclear fusion. If this occurred, there would be a large increase in temperature, over and above that produced by the heating effect of the current. Fleischmann and Pons reported just such an effect.

Fleischmann was reluctant to reveal too many details of their work lest it prejudice their patent application. He did, however, collaborate with Harwell, the British Atomic Energy Research Establishment, who were best suited to replicate cold fusion. They found no evidence of fusion, no excess heat. Whereas positive results had been obtained by several leading institutions, they were subsequently withdrawn when errors were detected. Despite such setbacks Fleischmann remained convinced of the essential soundness of his work with Pons.

From 1990 Fleischmann's work was mainly supported by Minora Toyoda, a Japanese businessman; he was based in Sophia Antipolis, a research center established by Toyoda outside Nice, France, concerned with the development of future technology. He retired in 1995 and returned to England.