Ithamar was the youngest of the four sons of Aaron (Ex. 6:23). He was consecrated as a priest along with his father and three brothers - Nadab, Abihu and Eleazar (Ex. 28:1). His job was to number the articles collected for the Tabernacle (Ex.38:21) and to supervise two priestly families, the Gershonites and the Merarites (Num. 4:21-33).
The priestly family founded by him included the high priest Eli and his descendants. Although Ithamar's family eventually lost the high priesthood, it continued as a priestly family after the Babylonian captivity (Ezra 8:2).