Einthoven, the son of a physician, was born at Semarang on the Indonesian island of Java and educated at the University of Utrecht in the Netherlands, where he gained his MD in 1885. In the following year he moved to Leiden as professor of physiology.

As early as 1887 the English physiologist Augustus Waller had recorded electric currents generated by the heart. He had used the capillary electrometer invented by Gabriel Lippmann in 1873, which – although sensitive to changes of a millivolt – turned out to be too complicated and inaccurate for general use. In 1901 Einthoven first described a recording system using a string galvanometer, which he claimed would overcome the inadequacies of Waller's device.

A string galvanometer consists of a fine wire thread stretched between the poles of a magnet. When carrying a current it is displaced at right angles to the directions of the magnetic lines of force to an extent proportional to the strength of the current. By linking this up to an optical system the movement of the wire can be magnified and photographically recorded. As the differences in potential developed in the heart are conducted to different parts of the body it was possible to lead the current from the hands and feet to the recording instrument to obtain a curve that was later called an electrocardiogram (ECG).

Having demonstrated the potentiality of such a machine, two further problems needed solution. Einthoven first had to standardize his ECG so that different machines or two recordings of the same machine would produce comparable readings. It was therefore later established that a 1 millivolt potential would deflect a recording stylus 1 centimeter on standardized paper. The second problem was how to interpret such a curve in order to distinguish normal readings from recordings of diseased hearts. By 1913 Einthoven had worked out the interpretation of the normal tracing and, by correlating abnormal readings with specific cardiac defects identified at post mortem, was able to use the ECG as a diagnostic tool.

For his development of the electrocardiogram Einthoven was awarded the 1924 Nobel Prize for physiology or medicine.