John Alexander MacWilliam (31 July 1857 – 13 January 1937), a physiologist at the University of Aberdeen in the late nineteenth and early twentieth centuries, was a pioneer in the field of cardiac electrophysiology. He spent many years studying ventricular fibrillation, and was the first person to propose that ventricular fibrillation was the most common cause of sudden death - and that fibrillation could be terminated (and life potentially saved) by a series of induction shocks to the heart. He was the first to accurately describe the condition of arrhythmia (irregular heartbeat), and he suggested transthoracic pacing to treat transient asystole (cardiac arrest). Although his work was recognised within his lifetime, it was not until many decades later that it laid the foundations for developments in the understanding and treatment of life-threatening heart conditions, such as in the artificial cardiac pacemaker. MacWilliam was appointed Regius Professor of the Institutes of Medicine (later Physiology) at the University of Aberdeen at the age of 29 in 1886, and remained in that post for 41 years until his retirement in 1927.

MacWilliam was born 31 July 1857 at Kiltarlity, near Beauly, Inverness-shire in Scotland, where his father was farmer at Culmill farm. His parents were William McWilliam (1814 - 1888) and his wife Isabella Cumming (1816 - 1887) who had moved around 1850 from the neighbouring parishes of Inveravon (sometimes spelled Inveraven) and Knockando on Speyside to the farm at Culmill. His mother Isabella Cumming was the youngest daughter of John and Helen Cumming, founders of the Cardhu whisky distillery on Speyside.

John Alexander had two siblings. His elder brother, William Lewis McWilliam, was born at Kiltarlity in 1855. He stayed as farmer at Culmill and married Mary Burns. They had no children. William was a respected farmer and an Inverness-shire County Councillor, who was also chairman of the local schools committee and chairman of the Parish Council for over 30 years. He died in 1936, aged 81. His sister, Isabella Helen McWilliam, was born 12.10.1859 and died in infanthood – at the age of just 16 months.

Around the late 1880s, he chose to alter the spelling of his name to MacWilliam rather than McWilliam. His published papers until 1889 are under the McWilliam spelling.

MacWilliam was educated at Kiltarlity parish school until moving on to Aberdeen University at the age of 17 in 1874. At Aberdeen he studied in the Arts faculty for two years before changing course to study medicine. He graduated M.B, C.M. in 1880, and was awarded the John Murray medal in that year for outstanding achievement.

After postgraduate work at the University of Edinburgh and at University College, London, MacWilliam worked with physiologists Hugo Kronecker at Bern, and Carl Ludwig, director of the Physiological Institute at the University of Leipzig. In 1847 Ludwig had invented the Kymograph (a mechanical instrument to record heartbeat and other muscle contractions or movements), but of course this was still before the invention of the electrocardiograph. MacWilliam began his research with Ludwig on the hearts of cold-blooded animals such as eels, fish and frogs, and noted a phylogenic similarity between the electrical behaviour of hearts progressing from these creatures to man. He logically extended his experiments on ventricular fibrillation in lower animals to humans, surmising that this arrhythmia was the cause of sudden death. After returning from the continent, MacWilliam received his M.D. degree (with highest honours) from Aberdeen University in 1882 for his thesis: Part 1 – "On the cardiac muscular fibre in various animals", Part 2 – "On the diaphragmatic fibre in various animals".

In the four years from 1883 till 1886 MacWilliam worked primarily at University College London, as demonstrator under Edward Albert Sharpey-Schafer (previously Schäfer), and working closely with eminent contemporaries in physiology there (W.D. Halliburton and Ernest Starling), and with inspiration and guidance from W.H. Gaskell at King's College, Cambridge. In 1886 MacWilliam was appointed Regius Professor of the Institutes of Medicine at the University of Aberdeen – at just 29 years old – succeeding Professor William Stirling. The role was subsequently retitled as Regius Professor of Physiology.

During his time at University College, MacWilliam studied the structure of the muscular fibres of the diaphragm and of the heart. The results of his research were communicated to the Royal Society in a paper "The structure and rhythm of the heart of the eel" – with the results of experiments extending over two years on the origin and conduction of the wave of contraction which sweeps over the eel's heart, the different ways in which the wave could be blocked, and the manner in which its direction could be changed. From 1885 onwards, there followed three more years of systematic investigation. He extended his experiments to the hearts of mammals, most often cats, and found that all he had observed in the heart of the (cold-blooded) eel was also to be seen in the mammalian heart. He studied the condition known then as "Herz-delirium" or "ventricular fibrillation", which became of importance to clinicians in the opening decades of the twentieth century. He sought to explain the manner in which this incoordination in the action of the heart's muscle was brought about and the manner in which normal contraction could be restored. He found that there was a "certain area along the septum" where inhibition of the wave of contraction could be easily produced. He observed the condition which became known as "auricular flutter", and concluded that fibrillation was a disorder of muscle, not of nerve. He also described the basic elements of cardiopulmonary resuscitation - commonly abbreviated as CPR - (ventilation and cardiac compression) in keeping his experimental animals alive.