Keynes's experience at Versailles was influential in shaping his future outlook, yet it was not a successful one for him. Keynes's main interest had been in trying to prevent Germany's compensation payments being set so high it would traumatise innocent German people, damage the nation's ability to pay and sharply limit her ability to buy exports from other countries — thus hurting not just Germany's own economy but that of the wider world. Unfortunately for Keynes, conservative powers in the coalition that emerged from the 1918 coupon election were able to ensure both Keynes himself and the Treasury were largely excluded from formal high-level talks concerning reparations. Their place was taken by the Heavenly Twins - the Judge Lord Sumner and the Banker Lord Cunliffe whose nickname derived from the "astronomically" high war compensation they wanted to demand from Germany. Keynes was forced to try to exert influence mostly from behind the scenes.

The three principal players at Versailles were Britain's Lloyd George, France's Clemenceau and America's President Wilson. It was only Lloyd George to whom Keynes had much direct access; until the 1918 election he had some sympathy with Keynes's view but while campaigning had found his speeches were only well-received by the public if he promised to harshly punish Germany, and had therefore committed to extracting high payments. Lloyd George did however win some loyalty from Keynes with his actions at the Paris conference by intervening against the French to ensure the dispatch of much-needed food supplies to German civilians. Clemenceau also pushed for high reparations; generally France argued for an even more severe settlement than Britain. Wilson initially favoured relatively lenient treatment of Germany – he feared too harsh conditions could foment the rise of extremism, and wanted Germany to be left sufficient capital to pay for imports. To Keynes's dismay, Lloyd George and Clemenceau were able to pressure Wilson to agree to very high repayments being imposed. Towards the end of the conference, Keynes came up with a plan that he argued would not only help Germany and other impoverished central European powers but also be good for the world economy as a whole. It involved the writing down of war debts which would have the effect of increasing international trade all round. Lloyd George agreed it might be acceptable to the British electorate. However America was against it, the US then being the largest creditor and by this time Wilson had started to believe in the merits of a harsh peace as a warning to future aggressors. So despite his best efforts, the end result of the conference was a treaty which disgusted Keynes both on moral and economic grounds, and led to his resignation from the Treasury.

In June 1919 he turned down an offer to become chairman of the British Bank of Northern Commerce, a job that promised a salary of £2000 in return for a morning per week of work.

Keynes's analysis on the predicted damaging effects of the treaty appeared in the highly influential book, The Economic Consequences of the Peace, published in 1919. This work has been described as Keynes's best book, where he was able to bring all his gifts to bear — his passion as well as his skill as an economist. In addition to economic analysis, the book contained pleas to the reader's sense of compassion:

Also present was striking imagery such as "...that year by year Germany must be kept impoverished and her children starved and crippled..." along with bold predictions which were later justified by events:

Keynes's predictions of disaster were borne out when the German economy suffered the hyperinflation of 1923, and again by the collapse of the Weimar Republic and the outbreak of World War II. Only a fraction of reparations were ever paid. The Economic Consequences of the Peace gained Keynes international fame, but also caused him to be regarded as anti-establishment – it was not until after the outbreak of World War II that Keynes was offered a directorship of a major British Bank, or an acceptable offer to return to government with a formal job. Keynes was still able to influence policy making however – through his network of contacts, his published works and by serving on government committees, including attending high-level policy meetings as a consultant.

Keynes had completed his Treatise on Probability before the war, but published it in 1921. The work was a notable contribution to the philosophical and mathematical underpinnings of probability theory, championing the important view that probabilities were no more or less than truth values intermediate between simple truth and falsity. Keynes developed the first upper-lower probabilistic interval approach to probability in chapters 15 and 17 of this book, as well as having developed the first decision weight approach with his conventional coefficient of risk and weight, c, in chapter 26. In addition to his academic work, the 1920s saw Keynes active as a journalist selling his work internationally and working in London as a financial consultant. In 1924 Keynes wrote an obituary for his former tutor Alfred Marshall which Schumpeter called "the most brilliant life of a man of science I have ever read." Marshall's widow was "entranced" by the memorial, while Lytton Strachey rated it as one of Keynes's "best works".

In 1922 Keynes continued to advocate reduction of German reparations with A Revision of the Treaty. He attacked the post World War I deflation policies with A Tract on Monetary Reform in 1923 – a trenchant argument that countries should target stability of domestic prices, avoiding deflation even at the cost of allowing their currency to depreciate. Britain suffered from high unemployment through most of the 1920s, leading Keynes to recommend the depreciation of sterling to boost jobs by making British exports more affordable. From 1924 he was also advocating a fiscal response, where the government could create jobs by spending on public works. During the 20's Keynes' pro stimulus views had only limited effect on policy makers and mainstream academic opinion — according to Minsky one reason was that at this time his theoretical justification was "muddled" . The Tract had also called for an end to the gold standard. Keynes advised it was no longer a net benefit for countries such as Britain to participate in the gold standard, as it ran counter to the need for domestic policy autonomy. It could force countries to pursue deflationary policies at exactly the time when expansionary measures were called for to address rising unemployment. The Treasury and Bank of England were still in favour of the gold standard and in 1925 they were able to convince the then Chancellor Winston Churchill to re-establish it, which had a depressing effect on British industry. Keynes responded by writing The Economic Consequences of Mr. Churchill and continued to argue against the gold standard until Britain finally abandoned it in 1931.

Keynes had begun a theoretical work to examine the relationship between unemployment, money and prices back in the 1920s. The work, Treatise on Money, was published in 1930 in two volumes. A central idea of the work was that if the amount of money being saved exceeds the amount being invested – which can happen if interest rates are too high – then unemployment will rise. This is in part a result of people not wanting to spend too high a proportion of what employers pay out, making it difficult, in aggregate, for employers to make a profit. At the height of the Great Depression, in 1933, Keynes published The Means to Prosperity, which contained specific policy recommendations for tackling unemployment in a global recession, chiefly counter cyclical public spending. The Means to Prosperity contains one of the first mentions of the multiplier effect. While it was addressed chiefly to the British Government, it also contained advice for other nations affected by the global recession. A copy was sent to the newly elected President Roosevelt and other world leaders. The work was taken seriously by both the American and British governments, and according to Skidelsky, helped pave the way for the later acceptance of Keynesian ideas, though it had little immediate practical influence. In the 1933 London Economic Conference opinions remained too diverse for a unified course of action to be agreed upon.