Due to the dry climate of Egypt, it was noted early on that bodies would naturally dessicate. The earliest bodies we have were simply buried, allowing the chances of the dry sands to preserve as much of the body as possible. Pre-dynastic burials in sand have been uncovered, sometimes yielding well-preserved bodies. However, what we think of as true mummification was practiced from at least the 4th Dynasty, according to John F. Nunn (Ancient Egyptian Medicine).

I believe mummification as a practice continued up until the advent of Coptic Christianity, although the technique reached its pinnacle during the 21st century. Middle and New Kingdom mummification practices were available to many outside of the royal court, although it is doubtful true commoners were ever mummified.

A variety of causes led to declines in the quality of mummification, typically economic in origin.