We have a number of science fiction works that take place in an Empire of Man/Empire of Mankind
- Warhammer 40k universe
- Jerry Pournelle Co-Dominium series
- Asimov - original Foundation trilogy
to name just 3.
For the purposes of the question, the Empire in any answer has to be at a galactic or at least extra-solar level. Asimov wrote the first Foundation stories in the 1940's, and is the oldest that I am aware of, but are there earlier works of Science Fiction that contain an Empire of Man/Empire of Mankind?