He has been called the most famous Orthodox Jewish legal authority of the twentieth century[2] and his rulings are often referenced in contemporary rabbinic literature.

Feinstein served as president of the Union of Orthodox Rabbis, Chairman of the Council of the Moetzes Gedolei HaTorah of the Agudath Israel of America, and head of Mesivtha Tifereth Jerusalem in New York.

Feinstein was president of the Union of Orthodox Rabbis of the United States and Canada, and chaired the Moetzes Gedolei HaTorah of Agudath Israel of America from the 1960s until his death.

Feinstein was recognized by many as the preeminent halakhic authority (posek) of his generation; ruling on issues of Jewish law as they pertain to modern times.

[12] Owing to his prominence as an adjudicator of Jewish law, Feinstein was often asked to rule on very difficult questions, whereupon he often employed a number of innovative and controversial theories in arriving at his decisions.

Soon after arriving in the United States, he established a reputation for handling business and labor disputes, writing about strikes, seniority, and fair competition.

[13] As one of the prominent leaders of American Orthodoxy, Feinstein issued opinions that clearly distanced his community from Conservative and Reform Judaism.

[c] According to Rabbi Dovid Cohen of Brooklyn, Feinstein attributed such comments to students as a way of politely rejecting statements by rishonim while still retaining full reverence for them as religious leaders of earlier generations.