Haldane was born in Edinburgh, the son of the physiologist John Scott Haldane and his wife Louisa Kathleen Haldane, and descended from Scottish aristocrats (see Haldane family). His younger sister Naomi Mitchison would later become a writer. Haldane was educated at Dragon School, Eton College (which he hated) and at New College, Oxford.

During the First World War, Haldane served with the Black Watch in France and Iraq. He was initially Bombing Officer for the 3rd Battalion before becoming a Trench Mortar Officer in the 1st. Whilst in the army, he became a socialist, writing "If I live to see an England in which socialism has made the occupation of a grocer as honourable as that of a soldier, I shall die happy".

Between 1919 and 1922 he was a fellow of New College, then moved to Cambridge University until 1932. He then moved to University College, London where he spent most of the remainder of his academic career. In the late 1950s he moved to India. The move was ostensibly a protest against the Suez War, but in reality had been on the cards for some while.

In 1924 Haldane met Charlotte Burghes (nee Franken) and the two later married. To do so Charlotte divorced her husband Jack Burghes, causing some controversy.

In 1925, Briggs and Haldane gave a mathematically superior derivation of the 1913 Michaelis-Menten equation, still a mainstay of enzyme kinetics in biochemistry. The original authors made the assumption that enzyme (catalyst) and substrate (reactant) are in quasi-equilibrium with their complex, which then dissociates to yield product and free enzyme.

Briggs and Haldane arrived at an equation of the same algebraic form, but with a better interpretation of the Michaelis constant. Their derivation makes the more realistic assumption that the intermediate complex(es) are initially in a quasi-steady-state as substrate is consumed and product is formed.

Haldane's famous book, The Causes of Evolution (1932), was a major work of what came to be known as the "modern evolutionary synthesis", reestablishing natural selection as the premier mechanism of evolution by explaining it in terms of the mathematical consequences of Mendelian genetics. The other major figures were R. A. Fisher and Sewall Wright.

Haldane was a keen experimenter, willing to expose himself to danger to obtain data. One experiment involving elevated levels of oxygen saturation triggered a fit which resulted in him suffering crushed vertebrae. In his decompression chamber experiments, he and his volunteers suffered perforated eardrums, but, as Haldane stated in What is Life, "the drum generally heals up; and if a hole remains in it, although one is somewhat deaf, one can blow tobacco smoke out of the ear in question, which is a social accomplishment."

He was also a great science populariser, and was perhaps the Isaac Asimov, Stephen Jay Gould, or Richard Dawkins of his day. His essay, 'Daedalus or Science and the Future' (1923), was remarkable in predicting many scientific advances but has been criticized for presenting a too idealistic view of scientific progress.

Haldane was himself a very idealistic man, and in his youth was a devoted Communist and author of many articles in The Daily Worker. Events in the Soviet Union, such as the rise of the anti-Mendelian agronomist Trofim Lysenko and the crimes of Stalin, caused him to break with the Communist Party later in life. He joined the Communist party in 1937 but left in 1950, shortly after having toyed with standing for Parliament as a Communist Party candidate.

He is also known for an observation from his essay, On Being the Right Size, which Jane Jacobs and others have since referred to as Haldane's principle. This is that sheer size very often defines what bodily equipment an animal must have: "Insects, being so small, do not have oxygen-carrying bloodstreams. What little oxygen their cells require can be absorbed by simple diffusion of air through their bodies. But being larger means an animal must take on complicated oxygen pumping and distributing systems to reach all the cells." The conceptual metaphor to animal body complexity has been of use in energy economics and secession ideas.

Haldane was friends with the author Aldous Huxley, and was the basis for the biologist Shearwater in Huxley's novel Antic Hay. Ideas from Haldane's Daedalus, such as ectogenesis (the development of fetuses in artificial wombs), also influenced Huxley's Brave New World.

Haldane had many students, the most famous of whom, John Maynard Smith, was perhaps also the one most like himself.

Quotations

"The wise man regulates his conduct by the theories both of religion and science. But he regards these theories not as statements of ultimate fact but as art-forms."

"Scientific education and religious education are incompatible. The clergy have ceased to interfere with education at the advanced state, with which I am directly concerned, but they have still got control of that of children. This means that the children have to learn about Adam and Noah instead of Evolution; about David who killed Goliath, instead of Koch who killed cholera; about Christ's ascent into heaven instead of Montgolfier's and Wright's. Worse than that, they are taught that it is a virtue to accept statements without adequate evidence, which leaves them a prey to quacks of every kind in later life, and makes it very difficult for them to accept the methods of thought which are successful in science."

"While I do not suggest that humanity will ever be able to dispense with its martyrs, I cannot avoid the suspicion that with a little more thought and a little less belief their number may be substantially reduced."

"In fact, words are well adapted for description and the arousing of emotion, but for many kinds of precise thought other symbols are much better."

"A fairly bright boy is far more intelligent and far better company than the average adult."

"Now my own suspicion is that the Universe is not only queerer than we suppose, but queerer than we can suppose."