They were originally mostly or all at the beginning, as in several minutes of credits before the movie started. There were occasional exceptions where they had fewer or just a title screen prior to the 70s, but the vast majority had several minutes of credits before the movie started.

Star Wars kicked off popularity of pushing the credits to the end of the movie. Again, not the first, but the start of the popularity. Pretty sure Lucas received a fine for doing it as well.

Since then most movies tend to have a few credits at the beginning and the majority at the end. In my opinion this was inevitable. Star Wars had two good reasons to move them back from.my perspective, it let the story start right away, and listing everyone involved with the special effects would have taken forever. The light credits, especially those overlaying the opening scenes is a lot better than the wall of text that was displayed before movies even started prior to the late 70s.