Rav Mordechai Khamus HaCohen, known as Amiyes, was born in Djerba in 5646 or 5648 (around 1886-1888), the son of Aziza and Rabbi Yitzchak HaCohen, a goldsmith descended from Rabbi Shaul HaCohen. His brothers were Rabbi Yaakov HaCohen, author of Bechor Yaakov, and Rabbi Yosef HaCohen, rabbi of Sfax. Tradition relates that his mother prayed for him to become a great man in Israel.

He studied under Rabbi Yosef Berrebi. Married at sixteen, in 5663, he walked seven kilometres every day to learn from Rabbi Moshe Zaken Mazuz, head of the Djerba rabbinical court. He was nicknamed "Amiyes", the razor, for the sharpness of his mind. At seventeen he was already publishing articles in the journal HaMe'assef and corresponding with rabbis in America, Russia and the Land of Israel; at eighteen he was known as an authority on the laws of niddah. Torah was so precious to him that he reportedly allowed himself only three hours of rest a day.

In 5670 Rabbi Moshe Zaken Mazuz brought him onto the Djerba rabbinical court, and in 5675 he became head of the island's rabbis. He reorganised the community, enlarged the court and enacted ordinances to strengthen observance. Together with Rabbi Moshe Khalfon HaCohen he fought the opening of an Alliance Israélite Universelle school in Djerba in 5697. In 5698 he stepped down and moved to Tunis. In 5710 Chief Rabbi David Ben Baron asked him to take his place as head of the great rabbinical court, and in 5718 he succeeded him as Chief Rabbi of Tunisia, the first Djerba-born rabbi to hold the post.

His good relations with President Habib Bourguiba eased his leadership of the community; when the president fell ill, he had prayers said for his recovery in synagogues throughout the country. He also actively helped North African Jews emigrate to Israel, at times at risk to his own standing.

He passed away in Tunis on Shavuot, 6 Sivan 5734 (27 May 1974), at the age of eighty-eight according to Hevrat Pinto. Works: Gedulat Mordechai, responsa in three volumes published after his death; Doresh Tov LeAmo, sermons; and VeErech HaKohen, halachic studies printed as an appendix to the latter.