No written Mandarin was not invented in the 20th century. There was some pronunciation and orthographic standardization, but written Mandarin has been the lingua franca of at least the literate class since at least the Song Dynasty, maybe even earlier, and written Mandarin itself extends back to the Tang Dynasty.

Here's for example here's the Southern Song philosopher Zhu Xi complaining about "kids these days" in written Mandarin (these are his original words, not translated into modern Mandarin), with modern Mandarin spelling marked in square brackets.

> 今学者不见有奋发底[的]意思，只是如此悠悠地过；今日见他是如此，明日见他亦是如此。《朱子语类·朱子十八》

And here's the Northern Song Cheng brothers complaining about Buddhism (again in the original)

> 佛学只是以生死恐动人，可怪二千年来无一人觉此，是被他恐动也[this 也 is purely for effect, same as it is occasionally done in modern Mandarin]！《二程遗书·卷一》