Rabbi Elazar HaModai was a Tanna of the third generation, in the time of Rabbi Akiva. He lived on "Mount Modi'im", most likely Modiin, not far from Jerusalem, hence his name. From the accounts the Talmud gives about him, he was in all likelihood one of the disciples of Rabban Yochanan ben Zakkai, and he lived a long life; he was a contemporary of Rabbi Yehoshua ben Chananya, Rabbi Eliezer ben Hyrcanus, Rabban Gamliel of Yavneh, Rabbi Tarfon and Rabbi Yishmael.

The Mishnah quotes only one teaching of his, in Pirkei Avot: "One who profanes sacred things, despises the festivals, breaks the covenant of Avraham our father, interprets the Torah contrary to its meaning and shames his fellow in public, even if he has Torah and good deeds, has no share in the World to Come." He excelled above all in aggadah, and it is in the Midrashim and the Talmud that his words are most often recorded. The Talmud relates that, sitting with Rabbi Tarfon, Rabbi Yishmael and the Sages, he stated that the manna that fell for Israel rose sixty cubits high; Rabbi Tarfon rebuked him, "Modai, how long will you keep piling up words?", and he answered, "Rabbi, I am expounding a verse!", and went on to explain his proof at length. Rabban Gamliel valued his interpretations and said, "We still need the Modai."

He lived through the Hadrianic persecutions and the Bar Kochba revolt, and many of his teachings, such as the one on the manna, "the portion of the day on its day", from which he taught that "He who creates the day creates its sustenance", are understood in that context. According to the Jerusalem Talmud, he was in besieged Betar, where he was the spiritual leader of the defenders, and Bar Kochba was, according to the Midrash, his nephew. There he fasted and prayed every day that the city should not fall, and it was known that as long as his merit protected the besieged, Betar would not be taken. A Samaritan slipped into the city, pretended to whisper a message to him, and then, when questioned, implied that Rabbi Elazar meant to hand the city over to the Romans. Bar Kochba questioned the sage, who, absorbed in prayer, had heard nothing and denied it; not believing him, Bar Kochba kicked him, and the aged sage, weakened by fasting, died. With him the divine protection of the city departed, and Betar fell shortly afterwards, on the 9th of Av. Some scholars identify him with the "Elazar HaKohen" whose name appears on Bar Kochba's coins.

Tradition places his tomb in the Upper Galilee, near the village of Nevoraya (Nabratein), beside that of Yaakov of Kfar Nevoraya; this tradition first appears in writings of the late 14th century (the Florence Scroll) and the late 15th century, long after the time of the Tanna, who died in Betar according to the Talmud.