The congregation was founded by 33 mainly German Jews who assembled for services in April 1845 in a rented hall near Grand and Clinton Streets in Manhattan's Lower East Side.

[2] Radical departures from Orthodox religious practice were soon introduced to Temple Emanu-El, setting precedents that proclaimed the principles of "classical" Reform Judaism in America.

In 1848, the German vernacular spoken by the congregants replaced the traditional liturgical language of Hebrew in prayer books.

In 1853, the tradition of calling congregants for aliyot was abolished (but retained for bar mitzvah ceremonies), leaving the reading of the Torah exclusively to the presiding rabbi.

Most controversially, mixed seating was adopted, allowing families to sit together, instead of segregating the sexes on opposite sides of a mechitza.

In 1868, Emanu-El erected a new building for the first time, a Moorish Revival structure by Leopold Eidlitz, assisted by Henry Fernbach at 43rd Street and 5th Avenue after raising about $650,000.

By the 1930s, Emanu-El began to absorb large numbers of Jews whose families had arrived in poverty from Eastern Europe and brought with them their Yiddish language and devoutly Orthodox religious heritage.

For the descendants of Eastern European immigrants, joining Temple Emanu-El often signified their upward mobility and progress in assimilating into American society.

However, the intake of these new congregants also helped to slow or halt, if not force, a limited retreat from, the 'rejectionist' attitude which "classical" Reform had espoused towards traditional ritual.