tl;dr: Prior to Mandarin, nothing was so widespread as Mandarin now.

1930's & Modern Standard Mandarin

The 1930's standardisation effort was by no means the first, nor really the most significant.

Prior to this, under KMT rule, in 1913 the Committee for the Unification of Pronunciation was formed in order to try to enforce a national standard. This led to the 1919 publication of the National Pronunciation Dictionary, but there were a number of practicality issues here. The idea behind the Committee was to create a form of Mandarin that took into account many different regional languages and pronunciations, and things that modern Standard Mandarin has lost, such as final -p -t -k -ʔ consonants ("entering tone" syllables), were incorporated. But this also meant that the form of Mandarin that they settled on was one which was not actually spoken by anyone. There were not native speaker teachers, and so each teacher throughout the country taught many of these "entering tone" words/syllables/morphemes as they would be in the individual teacher's language variety. This resulted in the obvious inconsistency, and this is what led to the shift in 1932. Concessions that were made to appease non-Mandarin speakers ultimately resulted in an un-teachable standard.

So it was then (1932) that the standard changed. People will tell you that it was the Beijing dialect that was made standard, but this is only partially true. It was educated speakers in Beijing whose pronunciation was made standard, but remember that these were people who had already had a huge amount of education in non-standard varieties or this earlier mixed standard, and so educated Beijingers weren't actually speaking Beijing dialect in the same way the average Beijinger was.

Along with the Nationalist era efforts, there was a push to vernacularise written language instead of using Classical Chinese. This was tied to the May 4th movement, and many important figures of the time such as Lu Xun, Hu Shih etc were involved in this, and often this push to vernacularise still resulted in a written standard that wasn't entirely in line with how the spoken languages were, but certainly closer than Classical Chinese was by then effectively nothing like what people spoke.

A bunch of stuff happened after that, and most of what PRC China did was stuff Nationalist China did or planned to do (including character simplification), but let's call this point in time "Modern Mandarin" and we can now look further backward in time.

The pre-Nationalist era language situation

So what was the standard before 1913? Actually it was still Mandarin in many cases (but not entirely, more on that in the next paragraph). Going all the way back to the 15th century, we have solid records of people learning Mandarin for the sake of interacting with the State. Early Mandarin was the de facto standard, and was even learned by foreigners; the Korean text Nogeoldae – a language-learning textbook that would look familiar to any language learner today – had many editions throughout the centuries. Whether the capital was in what's now Beijing or now Nanjing, a form of Mandarin was the primary method of communication. Prior to the Nationalist efforts in the early 1900s, a Nanjing-like Mandarin tended to be the main variety, but again with some innovations caused by non-locals speaking the language. But still Mandarin, albeit an earlier form of it that wouldn't have looked exactly like that of today.

However this only addresses a court standard and doesn't really address what was widely spoken, as you've asked about. So Mandarin is the standard today, or rather a single form of Mandarin is the standard today, but Mandarin is not strictly Standard Mandarin. Mandarin is a huge language with many many dialects, and spoken over a large area on it's own, unrelated to a position as the modern standard. It sounds very different the further you get from the Northeast, and varies widely in a lot of different ways, but in many cases it was still Mandarin that was spoken in those areas going way back. For people from these regions, obviously, learning a standard form of it would have been not too bad, or at least being understood would be quite feasible. But once we cross the Yangtze then things change quite abruptly.

As with the average person today, Mandarin isn't the day-to-day language (at least for people born before 1990, as a general marker). Wu areas like Shanghai, Suzhou, Zhejiang speak Wu. Xiang areas speak Xiang. Yue (Cantonese) areas speak Yue and so on. Mandarin is hugely in use and replacing these other languages because of the State apparatus. It's the education system succeeding at the task of enforcing this standard, and it's why you can now go pretty much anywhere in the PRC and speak Mandarin and be fine. But this is quite recent, and it's still quite easy to find people who understand Mandarin but don't really speak it much or at all. Prior to China's recent development, local schools would operate in the local language. You'd learn Standard Mandarin, but the teacher would speak to you in the language of that town (which again was still often some form of Mandarin if you were in the north). This was the case going into "classical" times as well, say, anything from the mid-Qing or before. (As a quick aside, a lot of the Nationalist's efforts at language reform and standardisation were also things the late Qing government tried to implement, but at that stage they were already not that effective at state rule, and so little of it had a lasting impact. Everyone's mostly just been doing the stuff the previous state had tried to do).

This gets us to Classical Chinese (CC). So what was spoken in the different regions was the regional language. So how did people communicate? CC is the solution. It's a hugely archaic written language, which also constantly changed from period to period, but still is identifiable as CC even if Xunzi was writing something that wouldn't have made much linguistic sense to Laozi due to differences in the language over the centuries. CC goes back to the earliest written records of Chinese. But for most of its history it was not the spoken language, and anyway there's evidence that even the oldest records, the Oracle Bones, were not even encoding a single language between inscriptions, but rather were probably reflecting many different languages and dialects.

Middle Chinese & before

Right around 601CE we have a document, a pronunciation dictionary, that marks what we call Middle Chinese. This is the common ancestor of Mandarin, Cantonese, Wu, Hakka, Xiang, Gan, Hui, Ping and Jin (and maybe I'm forgetting one). But not of Min/Hokkien/Fujianese/Taiwanese. Min marks an earlier branch off the family tree of Sinitic. There are also earlier branches, which I'll get to in a second. Min was already being spoken in Southeast China, and had been there for a long time, with significant diversity. A number of other non-Sinitic languages were also being spoken across the region of modern China, including probably Austronesian languages, Mon-Khmer languages, Tai languages, Hmong-Mien languages and other Tibeto-Burman varieties. Many of these were replaced or pushed out by the spread of Sinitic languages coming from Middle Chinese, and they probably left their marks on all of these Sinitic languages, contributing to the differences today between Mandarin and Cantonese (for example).

So some time around 600CE during the Tang we have something we can point to as a common ancestor of those non-Min Sinitic languages. They didn't just replace non-Sinitic languages though. They replaced Sinitic ones as well. In Sichuan there was a language generally just called Ba-Shu Chinese, named for two kingdoms in the area. We have tiny snippets of the language preserved in some local poetry from the period, but generally we don't know much about it and likely won't ever. Jiangdong (literally just "Eastern Yangtze") Chinese was another earlier branch along with Min and Ba-Shu Chinese, and this was spoken in the Yangtze Delta region. We also know nearly nothing about it. It's pointed too however as a substratum of Wu, along with probably She and some other languages.

Conclusion

This is getting long and I want to end it before the 10,000 limit so let's get to the answer: Prior to Mandarin, and really prior to 1990, what was spoken was the regional Sinitic languages until around a 1500 years ago or so (a really rough number just to help picture the situation; nothing magical happened 1500 years ago). Prior to that much greater areas of non-Sinitic languages, but many Tibeto-Burman and non-Sino-Tibetan languages. Many of these still exist in small communities in the Southeast and other areas. You get plenty of non-Sinitic islands in South China (and some in North China but not as many or as varied).

Mandarin having the dominance it has today is really a result of the modern education system and the significant national-level implementation of an education policy that prioritises Mandarin as the language of instruction in the classroom environment. This is the single most important thing leading to Mandarin having the influence it has today, and we only have to go a little bit before this was fully implemented to get to a point in history where regional languages were still the primary (and sometimes only) language in use. And even today in many rural or more remote areas, this is still the case.

Further reading

If you want an accessible and relatively current book on this very general subject, I recommend the following:

• Sun, Chaofen. Chinese: A Linguistic Introduction. Cambridge, UK: Cambridge University Press, 2006.

Some of the other staples are now pretty out of date, though still good, like Jerry Norman's Chinese, but Sun's is relatively in line with the current state of the field of historical Chinese linguistics.