Einthoven was graduated in medicine from the University of Utrecht and served as professor of physiology at the University of Leiden from 1886 until his death. In 1903 he devised the first string galvanometer, known as the Einthoven galvanometer; with this instrument he was able to measure the changes of electrical potential caused by contractions of the heart muscle and to record them graphically. He coined the term electrocardiogram for this process. Einthoven recognized differences in the records or tracings obtained from different kinds of heart disease. From 1908 to 1913 he studied the patterns of records of normal heart activity in order to gain precision in recognizing and interpreting deviations.

Einthoven continued to develop electrode arrangements, and the present-day standard limb leads were originally described and used by him. The clinical application of Einthoven's instrument was pioneered by the British physician Sir Thomas Lewis, with whom Einthoven maintained a long and fruitful correspondence.