Philipp Lenard was born in Pressburg (today's Bratislava), on 7 June 1862. The Lenard family had originally come from Tyrol in the 17th century, and Lenard's parents were German-speakers (Carpathian Germans). His father, Philipp von Lenardis (1812–1896), was a wine-merchant in Pressburg. His mother was Antonie Baumann (1831–1865). The young Lenard studied at the Pozsonyi Királyi Katolikus Főgymnasium (today Gamča), and as he writes it in his autobiography, this made a big impression on him (especially the personality of his Teacher, Virgil Klatt). In 1880, he studied physics and chemistry in Vienna and in Budapest. In 1882, Lenard left Budapest and returned to Pressburg, but in 1883, he moved to Heidelberg after his tender for an assistant's position in the University of Budapest was refused. In Heidelberg, he studied under the illustrious Robert Bunsen, interrupted by one semester in Berlin with Hermann von Helmholtz, and he obtained a doctoral degree in 1886. In 1887 he worked again in Budapest under Loránd Eötvös as a demonstrator. After posts at Aachen, Bonn, Breslau, Heidelberg (1896–1898), and Kiel (1898–1907), he returned finally to the University of Heidelberg in 1907 as the head of the Philipp Lenard Institute. In 1905, Lenard became a member of the Royal Swedish Academy of Sciences, and in 1907, of the Hungarian Academy of Sciences.

As a Physicist, Lenard's major contributions were in the study of cathode rays, which he began in 1888. Prior to his work, cathode rays were produced in primitive, partially evacuated glass tubes that had metallic electrodes in them, across which a high voltage could be placed. Cathode rays were difficult to study using this arrangement, because they were inside sealed glass tubes, difficult to access, and because the rays were in the presence of air molecules. Lenard overcame these problems by devising a method of making small metallic windows in the glass that were thick enough to be able to withstand the pressure differences, but thin enough to allow passage of the rays. Having made a window for the rays, he could pass them out into the laboratory, or, alternatively, into another chamber that was completely evacuated. These windows have come to be known as Lenard windows. He was able to conveniently detect the rays and measure their intensity by means of paper sheets coated with phosphorescent materials.

Lenard was the first person to study what has been termed the Lenard effect in 1892. This is the separation of electric charges accompanying the aerodynamic breakup of water drops. It is also known as spray electrification or the waterfall effect.

Lenard received the 1905 Nobel Prize for Physics in recognition of this work.

Lenard retired from Heidelberg University as professor of theoretical physics in 1931. He achieved emeritus status there, but he was expelled from his post by Allied occupation forces in 1945 when he was 83. He died in 1947 in Messelhausen, Germany.

Some measure of Lenard's views on certain Scientists may be deduced through examination of Lenard's book, Great Men in Science, A History of Scientific Progress, first published in 1933. The book was translated into English by Dr. H. Stafford Hatfield with an introduction by the famous scientist Edward Andrade of University College London (ironically, a Sephardic Jew himself) and was widely read in schools and universities after the Second World War. The individual Scientists selected for inclusion by Lenard do not include Einstein or Curie, nor any other twentieth century scientist. Andrade noted that "A strong individuality like that of the Writer of this book is bound to assert strongly individual judgements". The publisher included what now appears to be an equally remarkable note on page xix of the 1954 English edition: "While Professor Lenard's studies of the men of science who preceded him showed not only profound knowledge but also admirable balance, when it came to men of his own time he was apt to let his own strong views on contemporary matters sway his judgment. In his lifetime he would not consent to certain modifications that were proposed in the last study of the series".