According to Rashi and others here (and Shemos 37:2), the "mixed multitude" were already converted, and fully part of the Jewish nation.
According to Ibn Ezra and others here (same link), they were simply Egyptians who got "mixed in" with the Jewish nation leaving, but did not convert (or get the Torah, based on Ramban here), and were not counted with them (here).
Abarbanel to Shemos 12:43 suggests that there were some who were just added on, some who converted.
According to all opinions and they can be identified as the Asafsuf later on in Bamidbar 11:4, and likely many of them (if not all of them) died during this event 11:33. Pretty much any negative event that takes place in the desert (including the sin of the Golden Calf), there are opinions saying that it was the mixed multitude, and according to those opinions, any plagues that happened then would have also "killed them out".
Rashi (to Yehoshua 5:9) brings a Drash from Rav Yehuda (Moshe?) Hadarshan which implies that they entered the land of Israel, and according to Rashi and Abarbanel, it would seem simple that the sincere converts dis.