In the 1930s, Keynes spearheaded a revolution in economic thinking, overturning the older ideas of neoclassical economics that held that free markets would automatically provide full employment as long as workers were flexible in their wage demands. Keynes instead argued that aggregate demand determined the overall level of economic activity, and that inadequate aggregate demand could lead to prolonged periods of high unemployment. Following the outbreak of World War II, Keynes's ideas concerning economic policy were adopted by leading Western economies. During the 1950s and 1960s, the success of Keynesian economics resulted in almost all capitalist governments adopting its policy recommendations, promoting the cause of social liberalism.

Keynes's influence waned in the 1970s, partly as a result of problems that began to afflict the Anglo-American economies from the start of the decade, and partly because of critiques from Milton Friedman and other economists who were pessimistic about the ability of governments to regulate the business cycle with fiscal policy. However, the advent of the global financial crisis in 2007 has caused a resurgence in Keynesian thought. Keynesian economics has provided the theoretical underpinning for the economic policies of President Barack Obama of the United States, former Prime Minister Gordon Brown of the United Kingdom, and other global leaders to ease the late 2000s economic recession.

Keynes is widely considered to be the father of modern macroeconomics, and by various commentators such as economist John Sloman, the most influential economist of the 20th century. In 1999, Time magazine included Keynes in their list of the 100 most important and influential people of the 20th century, commenting that; "His radical idea that governments should spend money they don't have may have saved capitalism". In addition to being an economist, Keynes was also a civil servant, a director of the Bank of England, a patron of the arts and an art collector, a part of the Bloomsbury Group of intellectuals, an advisor to several charitable trusts, a writer, a private investor, and a farmer. In 1925, he married the Russian ballerina Lydia Lopokova, but remained childless.

John Maynard Keynes was born in Cambridge to a middle-class family. His father, John Neville Keynes, was an economist and a lecturer in moral sciences at the University of Cambridge and his mother Florence Ada Keynes a local social reformer. Keynes was the first born, and was followed by two more children - Geoffrey Keynes in 1887 and Margaret Neville Keynes in 1890.

According to economist and biographer Robert Skidelsky, Keynes' parents were loving and attentive, but not smotheringly so. They remained in the same house throughout their lives, where the children were always welcome to return. Keynes would receive considerable support from his father; including expert coaching to help him pass his scholarship exams and financial help both as a young man and when he was nearly wiped out at the onset of Great Depression in 1929. Keynes' mother made her children's interests her own, and according to Skidelsky, "because she could grow up with her children, they never outgrew home".

Keynes had his early education at home and in kindergarten. He attended St Faith's preparatory school as a day pupil from 1892-1897. Teachers described Keynes as brilliant, but on occasion, careless and lacking in determination. His health was often poor during this period, leading to several long absences.

Keynes won a scholarship to study at Eton, where he displayed talent in a wide range of subjects, particularly mathematics, classics and history. At Eton, Keynes experienced the first "love of his life" in Dan MacMillan, older brother to future Prime Minister Harold MacMillan. While there, he also embarked on at least one physically consumated sexual relationship, with a young Dilly Knox. Despite his middle-class background, Keynes mixed easily with upper-class pupils. In 1902 Keynes left Eton for King's College, Cambridge after receiving a scholarship for this also to study mathematics. The famous Alfred Marshall begged Keynes to become an economist, although Keynes's own inclinations drew him towards philosophy – especially the ethical system of G.E. Moore. Keynes was an active member of the semi-secretive Cambridge Apostles society, a debating club largely reserved for the brightest students. Like many members, Keynes retained a bond to the club after graduating and continued to attend occasional meetings throughout his life. Before leaving Cambridge, Keynes became the President of the Cambridge University Liberal Club. In May 1904 he received a first class B.A. in mathematics. Aside from a few months spent on holidays with family and friends, Keynes continued to involve himself with the university over the next two years. He took part in debates, further studied philosophy and attended economics lectures informally as a graduate student. He also studied for his 1905 Tripos and 1906 Civil Service exams.

The economist Harry Johnson wrote that the optimism imparted by Keynes's early life is key to understanding his later thinking. Keynes was always confident he could find a solution to whatever problem he turned his attention to, and retained a lasting faith in the ability of government officials to do good. Keynes's optimism was also cultural, in two senses – he was of the last generation raised by an empire still at the height of its power, in its own eyes and by much of the world (at least outwardly) seen as preeminent in both power and benevolence. Keynes was also of the last generation who felt entitled to govern by culture, rather than by expertise. According to Skidelsky, the sense of cultural unity current in Britain from the 19th century to the end of World War I provided a framework with which the well-educated could set various spheres of knowledge in relation to each other and to life, enabling them to confidently draw from different fields when addressing practical problems.

Keynes's Civil Service career began in October 1906, as a clerk in the India Office. He enjoyed his work at first, but by 1908 had become bored and resigned his position to return to Cambridge and work on probability theory, at first privately funded only by two Dons at the university – his father and the economist Arthur Pigou. In 1909 Keynes published his first professional economics article in the Economics Journal, about the effect of a recent global economic downturn on India Also in 1909, Keynes accepted a lectureship in economics funded personally by Alfred Marshall. Keynes's earnings rose further as he began to take on pupils for private tuition, and on being elected a fellow. In 1911 Keynes was made editor of the Economic Journal. By 1913 he had published his first book, Indian Currency and Finance. He was then appointed to the Royal Commission on Indian Currency and Finance – the same topic as his book – where Keynes showed considerable talent at applying economic theory to practical problems.

The British Government called on Keynes's expertise during the First World War. While he did not formally re-join the civil service in 1914, Keynes travelled to London at the government's request a few days before hostilities started. Bankers had been pushing for the suspension of specie payments – the convertibility of bank notes into gold – but with Keynes's help the Chancellor of the Exchequer (then Lloyd George) was persuaded that this would be a bad idea, as it would hurt the future reputation of the city if payments were suspended before absolutely necessary.