Who was the first leader to codify his laws?

We really cannot know the answer to this question, because too much of history has been lost. Legal codes appeared regularly in ancient times, with the oldest we have dating to about four thousand years ago. We have many other legal codes from ancient times. In fact, we have many from Rome alone.

During the Middle ages, emerging Germanic nations produced an abundance of legal codes. The link below to an article on Early Germanic Law lists about a dozen, with the earliest, the Code of Euric, dating to about 480 AD. This was only about four years after the most commonly used date for the fall of the West Roman Empire. An important and enduring legal code of the East Roman Empire, the Code of Justinian, appeared in the first decades of the Middle Ages.

There are doubtless many legal codes that have been lost. Quite possibly there are many more than have survived.

There are links below to articles on legal codes.