John Gray McKendrick was born in Old Machar, Aberdeen on 12 August 1841 and went on to study at the University of Aberdeen and the University of Edinburgh before graduating in 1864 as a MB ChB. He married Mary Souttar in 1867 and two of their children, John Souttar M’Kendrick and Anderson Gray M'Kendrick, would go on to become fellows of the Royal Society of Edinburgh in their own right. In 1869, he became the assistant to the Professor of Physiology at the University of Edinburgh, John Hughes Bennett, pursuing his own research into the nervous system and special senses. McKendrick went on to be elected as a Fellow of the Royal Society of Edinburgh in 1873, having been proposed by Sir William Turner, serving as a councillor and eventually the vice-president from 1894 until 1900.

He took up a post at the University of Glasgow in 1873, first as an extra-mural lecturer and then as Professor of 'Theory of Physic or Institutes of Medicine' in 1876. The name of his position was changed to Professor of Physiology in 1893.

McKendrick was a founder member of the Physiological Society and was Fullerian Professor of Physiology and Comparative Anatomy at the Royal Institution from 1881 to 1884. He resigned the latter post on 5 March 1884 due to ill health. In 1891 and 1895 he was invited to deliver the Royal Institution Christmas Lecture.

When he retired in 1910 he became Provost of Stonehaven. John Gray McKendrick FRSE died in Glasgow on 2 January 1926.