Who put together the Pesach Hagadah and the seder we have today?
The Mishneh provides some order, but who put together the Hagadah?
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Rav Menachem Mendel Kasher, says that part of it was compiled by the Anshei Knesses Hagedola. The Avudraham says that there were originally two versions one by Rav and Shmuel and one by Abaye and Rava. Rav Amram Gaon wrote some and so did Rav Saadiah Gaon. The Ma Nishtana is from a Mishna in Pesachim. Much of the Haggadah is from the Mishna, Gemara, and Midrashim.
If you're talking about the Maggid section, or basically the section of the Haggadah that precedes the meal, refer to this excellent article. It's mainly in Hebrew, and it would defeat the purpose for me to translate it.
In short, much of it comes from the Mishnah, and parts of it from various Siddurim such as Rav AMram Ga'on. Rav Sa'adiah Gaon, and some others. You'll also find a list of sources supporting th econcept of the mitzvah of telling the story of the Exodus.