Keynes remained convinced of the dangers of inflation to the end of his life, during World War II he argued strongly for policies that would minimise post war inflation.

Keynes's early romantic and sexual relationships were almost exclusively with men. Attitudes in the Bloomsbury Group, in which Keynes was avidly involved, were relaxed about homosexuality. One of his great loves was the artist Duncan Grant, whom he met in 1908, and he was also involved with the writer Lytton Strachey. Keynes was open about his bisexuality, and between 1901 to 1915, kept separate diaries in which he tabulated his sexual relationships.

In 1921 he fell "very much in love" with Lydia Lopokova, a well-known Russian ballerina, and one of the stars of Serge Diaghilev's Ballets Russes. They married in 1925, leading to the widely repeated couplet of unknown authorship: "Oh what a marriage of beauty and brains. The fair Lopokova and John Maynard Keynes". Their union was by all accounts happy, though childless – Lydia became pregnant in 1927 but miscarried. For the first years of the relationship, Keynes had maintained an affair with a younger man, Sebastian Sprott, in tandem with Lopokova, but he eventually chose Lopokova exclusively on marrying her. Among Keynes' Bloomsbury friends, Lopokova was, at least initially, subjected to criticism for her manners, mode of conversation and supposedly humble social origins - the latter of the causes being particularly noted in the letters of Vanessa and Clive Bell, and Virginia Woolf. In her novel Mrs Dalloway (1925), Woolf bases the character of Rezia Warren Smith on Lopokova. E.M. Forster would later write in contrition: 'How we all used to underestimate her'.

Keynes was ultimately a successful investor, building up a substantial private fortune. He was nearly wiped out following the Stock Market Crash of 1929 which he failed to foresee, but he soon recouped his fortune. At his death in 1946 Keynes's worth stood just short of £500,000 - equivalent to about £11 million ($16.5 million) in 2009. The sum had been amassed despite lavish support for various causes and his personal ethics which made him reluctant to sell on a falling market as he believed if too many did that it could deepen a slump.

Keynes built up a significant collection of fine art, including works by Paul Cézanne, Edgar Degas, Amadeo Modigliani, Georges Braque, Picasso, and Georges-Pierre Seurat. He enjoyed collecting books: for example, he collected and protected many of Isaac Newton's papers. It is in part on the basis of these papers that Keynes wrote of Newton as "the last alchemist." He was interested in literature in general and drama in particular and supported the Cambridge Arts Theatre financially, which allowed the institution, at least for a while, to become a major British stage outside of London.

Like several other notable British authors of his time, Keynes was a member of the Bloomsbury Group. Virginia Woolf's biographer tells an anecdote on how Virginia Woolf, Keynes and T. S. Eliot would discuss religion at a dinner party, in the context of their struggle against Victorian era morality. Keynes had attended church up to his teens, but by university he had become agnostic, which he remained until his death. At the end of the said dinner party, a disturbance reminded Keynes "of his theme", and he remarked that "the youth had no religion save Communism and this was worse than nothing." Marxism "was founded upon nothing better than a misunderstanding of Ricardo", and given time, he, Keynes, "would deal thoroughly with the Marxists" and other economists, to solve the economic problems their theories threatened to cause.

In 1931 Keynes went on to write the following on Marxism:

Keynes was a life-long member of the Liberal party, which until the 1920s had been one of the two main political parties in the United Kingdom, and as late as 1916 had often been the dominant power in government. Keynes had helped campaign for the Liberals at elections from as early as 1906, yet he always refused to run for office himself, despite being asked to do so on three separate occasions in 1920. From 1926 when Lloyd George became leader of the Liberals, Keynes took a major role in defining the party's economics policy, but by then the Liberals had been displaced into third party status by the Labour party.

Keynes's personal interest in Classical Opera and Dance led him to support the Royal Opera House at Covent Garden and the Ballet Company at Sadler's Wells. During the War as a member of CEMA (Council for the Encouragement of Music and the Arts) Keynes helped secure government funds to maintain both companies while their venues were shut. Following the War Keynes was instrumental in establishing the Arts Council of Great Britain and was the founding Chairman in 1946. Unsurprisingly from the start the two organisations that received the largest grant from the new body were the Royal Opera House and Sadler's Wells.