Mayow was born at Morvah in Cornwall and educated at Oxford University. He became a doctor of law in 1670 but then turned to medicine.

In 1674 he published his Tractatus quinque (Fifth Treatise) in which he came close to discovering the composite nature of the atmosphere. He showed that if a mouse is kept in a closed container over water then the air in the container will diminish in quantity, its properties change, and the water will rise up into the container. The same effect, he realized, could be produced by burning a candle. He further pointed out that combustion and respiration stopped before all the air was used up. He preceded Joseph Priestley and Antoine Lavoisier by about a hundred years with his discoveries relating to respiration and combustion.