The history of the Interslavic language is closely connected with Pan-Slavism, an ideology that believes that the Slavs are one nation and aims at their cultural and political unification. Along with the idea of a common Slavic state comes also the idea of a common Slavic language. From the beginning, the question has been what this language should look like.

A strong candidate has always been Russian, in terms of speakers the largest Slavic language by far. Nowadays, it is the native language of nearly half of the Slavs and spoken (almost) fluently by another 15–20%. Besides, Russia has always been a huge empire, a regional superpower, and it is the only Slavic country with an unbroken tradition of statehood. Not surprisingly, Russian has been advocated as a Pan-Slavic language mostly by Russian Pan-Slavists, who have always made it abundantly clear that Russian hegemony is a conditio sine qua non for a Slavic state, and this is precisely why the idea never gained much popularity among the smaller Slavic nations. Even though they recognise their cultural ties with Russia, they refuse be treated as „disfigured Russians” and most them do not have the best experience with Russian domination. In addition, Russian is not a simple language and it has numerous features that make it hard to understand for speakers of other Slavic languages.

Another popular candidate is Old Church Slavonic, the oldest written Slavic language, codified (or created, according to some) in the 9th century by the brothers St. Cyril and St. Method. This language has several advantages: it is very closely related to the common ancestor of the Slavic languages, and has a long tradition of use in Orthodox liturgy, thus being both a common ground and a common influence for many Slavic languages. It has the additional advantage of not being anybody's native language, so that it can easily be put forward as a neutral, international language similar to Latin or Sanskrit. In spite of these assets, however, Old Church Slavonic has several practical drawbacks as well. The fact that it was positioned as a sacred language has made it particularly resistant to change, so that even the later „redactions” are little more than the original with some couleur locale mixed in. As a result, Old Church Slavonic has an abundantly rich orthography containing several characters and sounds that have fallen out of use, an equally rich and therefore complicated grammar, and an extremely archaic vocabulary, featuring words that have not survived in any of the contemporary languages, yet lacking words for modern concepts. As early as the 17th century Juraj Križanić, arguably the first Pan-Slavist, noted that Old Church Slavonic had become too archaic to be feasible as a Pan-Slavic language. In other words, it would require some thorough modernisation, and this is where Interslavic comes in. Interslavic, one might say, begins where Old Church Slavonic ends.

Križanić was the first to actually describe a language, which he named Ruski in an effort to please the tsar, but which was in fact a mix of Russian Church Slavonic and Croatian, as well as elements from other Slavic languages, notably Polish. It was completed in 1666 and is often quoted as the first constructed Interslavic language. In later years, his example has been followed by many others. In the 19th century, when Pan-Slavism was at its peak, another ten projects by Czech, Slovak, Slovene and Croatian authors saw the light. Of these, most elaborate were: Universalis Lingua Slavica, published in 1826 by the Slovak Ján Herkeľ, a language based primarily on the West Slavic languages; Vsjeslovianьskyь, based on Church Slavonic and published in 1861 by the Czech Vaceslav Bambas; and Uzajemni Pravopis Slavjanski, published in 1865 by the Slovene Matija Majar-Ziljski. Whereas most projects were aimed at reviving and modernising (Old) Church Slavonic, Majar chose an entirely different approach. His project was based on the idea that Slavs could make themselves better understandable to other Slavs taking their own language as a starting point and then gradually modifying it. The first step in that process was altering the orthography, resulting in a generic („mutual”) Pan-Slavic orthography. Subsequently, he proposed a grammar that was based on linguistic comparison of the five major Slavic languages of those days: Russian, Polish, Czech, Serbian and Old Church Slavonic.

In the 20th century, several new projects appeared, mostly by Czech authors. Whereas previous projects had mostly been academic extrapolations, simplicity became an issue as well. Some projects, like Vsevolod Cheshikhin's Neposlava, Bohumil Holý's Slavski and Mark Hučko's Slovio, were heavily influenced by Esperanto. On the other hand, Ladislav Podmele's naturalistic Mežduslavjanski jezik from the 1950s was based mostly on Russian, probably due to the political reality of those days.

Although most of the afore-mentioned projects were elaborate and used in various publications, they were isolated efforts by individual people, and it seems unlikely that any of them has ever had a considerable following. The digital age opened many new possibilities for Interslavic. First of all, the growing possibilities for interaction with people in other countries, on mailing lists, internet fora etc. spawned a renewed interest in a language that would be understandable to Slavs of different nationalities. Secondly, Interslavic was no longer the domain of linguists and Pan-Slavists anymore, and everybody now had the chance to share his ideas on the Internet. And thirdly, thanks to the new means of communication, authors were no longer forced to work in isolation. Thus, in the early years of the 21th century, we witness a shift from solo projects towards collaborative projects, followed by a tendency towards their integration. The first collaborative project was called Slovo, active in 2001-2002 but discontinued afterwards.

Collaborative projects had some major advantages over solo projects. Previous projects, no matter how well-built, had always been coloured by the ethnicity of their authors, most of whom had traditionally been Czechs, Slovenes and Croats. For the first time, input from Russians, Poles, Serbs and Bulgarians started playing a role, too. Because people of different nationalities were in constant contact with each other and had access to the same sources, it became possible to do something that had never been done before, namely building an Interslavic dictionary. Besides, a language project that was worked on by a collective instead of an individual author, immediately had a rudimentary user community and testing range at its disposal.

The Slovianski project was started in March 2006, partly in response to the highly artificial character of Slovio, at the time the only active Slavic-based constructed language around. It was based on the same concept of linguistic comparison that had also been applied by Majar-Ziljski. All the living Slavic languages with at least 1 million speakers were taken into account, in such way that West, East and South Slavic were given equal weight. In the early days of the project, the authors experimented with different versions of the language. Slovianski-P (the letter P refers to the word pidgin or prosti, the Slavic word for „simple”), first proposed by Ondrej Rečnik and later adopted by Gabriel Svoboda, was intended to become a highly simplified language, built along similar lines as pidgin languages. Slovianski-N (the letter N stands for „naturalism”), initiated by Jan van Steenbergen and later elaborated by Igor Polyakov, was based on everything the Slavic languages have in common, including their case system. At last, Slovianski-S (the letter S stands for „schematicism”) was an effort of Gabriel Svoboda (GS-Slovianski) and Igor Polyakov (Slovjanskaj) to create a language that would combine schematicism with naturalism. All three of them worked as separate sister projects, or „dialects”, based on different grammar models, but sharing one orthography, phonology and vocabulary. Gradually, Slovianski-S and Slovianski-P were abandoned, and in 2009 it was decided that Slovianski-N would carry the name Slovianski, while Slovianski-P would continue to exist as a simplified version of it, useful for communication with Bulgarians, Macedonians and non-Slavs.

Meanwhile, separate efforts were undertaken to bridge the gap between Slovianski and Slovio by combining (a modified form of) Slovianski grammar with Slovio's larger dictionary. In 2008, a Russian, Hellerick, proposed Rozumio, about which he wrote:

„Rozumio is not a language, but rather an attempt to bring together two Slavic auxiliary conlangs Slovio and Slovianski. The language is based on Slovio, and yet can be considered a kind of Slovianski in its primitive form.”

The same idea was also picked up in February 2009 by Steeven Radzikowski, Andrej Moraczewski and Michal Borovička, when they started a new project, first called Slavju Slovio („Slavic Slovio”) and soon named Slovioski. Unlike Rozumio, Slovioski quickly developed into a separate language project. The original idea was to replace Slovio's grammar with a more naturalistic one, while basically sticking to the Slovio dictionary. Just like Slovianski, Slovioski had three versions, or „levels”: Prostij Slovioski, Srednij Slovioski and Polnij Slovioski. The latter was practically identical to Slovianski. In 2010, Srednij Slovioski was abolished, the Slovio dictionary abandoned, and Slovioski itself was renamed Interslavic.

For all versions of Slovianski and Slovioski goes that they were heavily influenced by the notion that an auxiliary language should be simple. Even Slovianski-N – in spite of it having gender and cases – had a highly simplified system for declining nouns and conjugating verbs. However, both projects showed an increasing tendency towards favouring natural solutions over simplicity. As a result, the naturalistic element ultimately prevailed in both projects. This shift towards naturalism was strengthened even more after a new project entered the scene: Novoslovienskij („Neoslavonic”), published in 2010 by Vojtěch Merunka. Unlike Slovianski and Slovioski, it was based directly on Old Church Slavonic, which it attempted to modernise by simplifying its orthography and expanding it with modern vocabulary taken from Slovianski, while preserving archaic elements like multiple past tenses and the dual.

All three projects collaborated closely with each other from that time, and in 2011 Medžuslovjanski („Interslavic”) was chosen as a common name. Fruits of this cooperation were, among other things, a common dictionary, a common news portal and a common wiki. As emphasis increasingly shifted from simplicity towards flexibility, Slovianski developed from a language with fixed rules into a more flexible language with an prototype orthography that could easily be converted according to one's needs, two different grammar levels and various possibilities for flavourisation. This multi-level grammar included a highly simplified grammar model, Slovianto, developed to replace both Slovianski-P and Prostij Slovioski, and a more advanced grammar that was based on Slovianski, Novosloviensky and various older projects, and aimed at integrating them. At that time, Slovioski ceased to exist as a separate project.

During the years to follow, Medžuslovjanski and Novoslovienskij (soon renamed Novoslověnsky) gradually grew closer to each other. Their user communities consisted largely of the same people, who often mixed elements from both projects. As a result, most differences between both projects vanished in a natural way.

On 1 and 2 June 2017, the First Conference on the Interslavic Language (CISLa 2017) took place in Staré Město near Uherské Hradiště in the Czech Republic, embedded in the fourth edition of the festival Days of Slavic Culture. With 64 participants from 12 different countries, the conference become a milestone in the history of Interslavic, because for the first time it was used during an official, public event. It was also a unique occasion for testing the language: most presentations and discussions were either held in Interslavic or translated consecutively into Interslavic, and this turned out to be sufficient for all Slavic participants – including Poles and Bulgarians – to understand virtually everything. A remarkable and rather unexpected side effect was that a few participants who had never learned Interslavic, suddenly started speaking it during the conference.

Another outcome of the conference was that the principal authors of both projects, Jan van Steenbergen and Vojtěch Merunka, decided to fully unite their projects, since working with two similar but not identical standards had become inconvenient. Immediately after the conference they formed a committee with three other fluent speakers of Interslavic (Roberto Lombino, Michał Swat and Pavel Skrylev), whose task it was to detect and eliminate all differences between both grammars. By the end of July this task had been accomplished, and as a result of the merger, the name of the language became Medžuslovjansky.