Born the son of a wine merchant in Pozsony (now Bratislava in Slovakia), Lenard studied at the universities of Budapest, Vienna, Berlin, and Heidelberg, where he obtained his doctorate. He taught at the universities of Bonn (1893), Breslau (1894), Aachen (1895), and Heidelberg (1896). In 1898 he was appointed professor of experimental physics at Kiel. He returned to Heidelberg in 1907, where he remained until his retirement in 1931.

Lenard's career falls naturally into two distinct periods. Before 1914 he made several major contributions to fundamental physics. In particular he investigated the photoelectric effect. It had been known for some time that light falling on certain metals would cause the emission of electrons. Starting in 1899 Lenard investigated why the effect could only be produced by ultraviolet or shortwave light. In the course of his experiments he established two anomalous results. He found that the speed with which the electron was emitted was a function of the wavelength of the light used – the shorter the wavelength the faster the electron. Increasing the intensity of the light did not affect the speed but did, surprisingly, increase the number of electrons emitted. It was left to Albert Einstein to explain the significance of these results in 1905 by linking them to the new quantum theory of Max Planck.

Lenard also did important work on cathode rays (electrons) for which he received the Nobel Prize for physics in 1905. He demonstrated how they could be induced to leave the evacuated tube in which they were produced, penetrate thin metal sheets, and travel a short distance in the air, which would become conducting. On the basis of this work he proposed a model of the atom in which it is made from ‘dynamids’, units of positive and negative charge. This was, however, soon superseded by the nuclear atom of Ernest Rutherford.

Lenard also seems to have come close to making two other discoveries. He almost discovered x-rays and felt that if he had not moved to Aachen in 1895 he would have been successful. He did in fact help their discoverer Wilhelm Röntgen with equipment – aid which, he argued, was never duly acknowledged. He also felt that J. J. Thomson had used some of his work without due recognition.

His suspicions of other workers were the first signs that Lenard was developing a somewhat idiosyncratic view of physics. The latter half of his career, from 1919, was spent arguing for the establishment of a new physics, a ‘German’ physics untainted with Jewish theories. Although Lenard was a German patriot who was deeply affected by Germany's defeat in 1918, he was not simply an anti-Semite. He attacked Einstein as a socialist, a pacifist, and, indeed, as a Jew; however his strongest abuse was directed toward him as a theoretical physicist. In 1920 Lenard organized a conference at Bad Neuheim to discuss relativity theory and attacked Einstein for somehow misleading people with a very abstract theory with little experimental support. He was also deeply upset by Einstein's dismissal of theories of the ether.

The only course for him was to develop a non-Jewish physics and to this end he produced a curious four-volume work, Deutsche Physik (1936–37; German Physics). Faced with the objection that science is international he replied, “It is false. Science like every other human product is racial and conditioned by blood.” The atmosphere produced by Lenard did much to cause the general exodus of scientists from Germany and to destroy creative science there for a generation.

Just why Lenard was transformed from a talented experimentalist into a bigoted and almost pathological crank is not clear. Germany's losing the war followed by the death of his son and the loss of all his savings in the postwar inflation no doubt contributed, but the ultimate source seems to have been his distaste, as an experimentalist, for the increasing mathematical abstraction introduced into physics by such scientists as Einstein.