In spring 1847 Marx and Engels joined the League of the Just, who were quickly convinced by the duo's ideas of "critical communism". At its First Congress in 2–9 June, the League tasked Engels with drafting a "profession of faith", but such a document was later deemed inappropriate for an open, non-confrontational organisation. Engels nevertheless wrote the "Draft of the Communist Confession of Faith", detailing the League's programme. A few months later, in October, Engels arrived at the League's Paris branch to find that Moses Hess had written an inadequate manifesto for the group, now called the League of Communists. In Hess's absence, Engels severely criticised this manifesto, and convinced the rest of the League to entrust him with drafting a new one. This became the draft Principles of Communism, described as "less of a credo and more of an exam paper."
On 23 November, just before the Communist League's Second Congress (29 November – 8 December 1847), Engels wrote to Marx, expressing his desire to eschew the catechism format in favour of the manifesto, because he felt it "must contain some history." On the 28th, Marx and Engels met at Ostend in Belgium, and a few days later, gathered at the Soho, London headquarters of the German Workers' Education Association to attend the Congress. Over the next ten days, intense debate raged between League functionaries; Marx eventually dominated the others and, overcoming "stiff and prolonged opposition", in Harold Laski's words, secured a majority for his programme. The League thus unanimously adopted a far more combative resolution than that at the First Congress in June. Marx (especially) and Engels were subsequently commissioned to draw up a manifesto for the League.
Upon returning to Brussels, Marx engaged in "ceaseless procrastination", according to his biographer Francis Wheen. Working only intermittently on the manifesto, he spent much of his time delivering lectures on political economy at the German Workers' Education Association, writing articles for the Deutsche-Brüsseler-Zeitung, and giving a long speech on free trade. Following this, he even spent a week (17–26 January 1848) in Ghent to establish a branch of the Democratic Association there. Subsequently, having not heard from Marx for nearly two months, the Central Committee of the Communist League sent him an ultimatum on 24 or 26 January, demanding he submit the completed manuscript by 1 February. This imposition spurred Marx on, who struggled to work without a deadline, and he seems to have rushed to finish the job in time. (For evidence of this, historian Eric Hobsbawm points to the absence of rough drafts, only one page of which survives.)
In all, the Manifesto was written over 6–7 weeks. Although Engels is credited as co-writer, the final draft was penned exclusively by Marx. From the 26 January letter, Laski infers that even the League considered Marx to be the sole draftsman (and that he was merely their agent, imminently replaceable). Further, Engels himself wrote in 1883 that "The basic thought running through the Manifesto ... belongs solely and exclusively to Marx." Although Laski doesn't disagree, he suggests that Engels underplays his own contribution with characteristic modesty, and points out the "close resemblance between its substance and that of the [Principles of Communism]". Laski argues that while writing the Manifesto, Marx drew from the "joint stock of ideas" he developed with Engels, "a kind of intellectual bank account upon which either could draw freely."