When Aaron and his four sons established Israel’s priestly order, Aaron became the high priest and his sons were the priests who assisted him. Ithamar was the youngest of the four sons (Exodus 28:1-4).
After the death of the two older sons, Eleazar and Ithamar received additional responsibilities. Eleazar, the senior of the two, had overall control of the Levites (Numbers 3:32), but Ithamar had specific responsibility for two of the three family divisions within the Levites (Numbers 4:28; Numbers 4:33). He had earlier supervised the building of the tabernacle (Exodus 38:21), and now he was the chief overseer of its maintenance (Numbers 4:24-33).