[3] He was the eldest of seven children of James Fleming DD (died 1879), a Congregational minister, and his wife Mary Ann, at Lancaster, Lancashire, and baptised on 11 February 1850.

Training to become an engineer was beyond the family's financial resources, but he reached his goal via a path that alternated education with paid employment.

Fleming enrolled for a BSc degree at University College London,[6] graduated in 1870, and studied under the mathematician Augustus De Morgan and the physicist George Carey Foster.

Financial problems again forced him to work for a living and in the summer of 1874 he became science master at Cheltenham College, a public school, earning £400 per year.

On 11 June 1887, he married[9] Clara Ripley (1856/7–1917), daughter of Walter Freake Pratt, a solicitor from Bath.

In 1884 Fleming joined University College London taking up the Chair of Electrical Technology, the first of its kind in England.

Although this offered great opportunities, he recalls in his autobiography that the only equipment provided to him was a blackboard and piece of chalk.

In 1904, working for the Marconi company to improve transatlantic radio reception, Fleming invented the first thermionic vacuum tube, the two-electrode diode, which he called the oscillation valve, for which he received a patent on 16 November.

[14][15] Fleming's diode was used in radio receivers and radars for many decades afterwards, until it was superseded by solid state electronic technology more than 50 years later.

In 1906, Lee De Forest of the US added a control "grid" to the valve to create an amplifying vacuum tube RF detector called the Audion, leading Fleming to accuse him of infringing his patents.

Fleming also contributed in the fields of photometry, electronics, wireless telegraphy (radio), and electrical measurements.

Fleming was awarded the IRE Medal of Honor in 1933 for "the conspicuous part he played in introducing physical and engineering principles into the radio art".

[16] On 27 November 2004 a Blue Plaque presented by the Institute of Physics was unveiled at the Norman Lockyer Observatory, Sidmouth, to mark 100 years since the invention of the thermionic radio valve.

[19] Fleming's archive spans 521 volumes and 12 boxes; it contains his laboratory notebooks, lecture notes, patent specifications, and correspondence.