He wrote several letters to Joseph Smith in Nauvoo, Illinois, in which he declared his desire to join the Church of Jesus Christ of Latter Day Saints.

[5] His efforts on behalf of the Mormons and the long time he spent living in the Mansion House in Nauvoo secured Bennett the confidence of Smith.

Contemporary sources indicate that Bennett used his trusted position as a doctor to allay fears of women he attempted to seduce by telling them that he could cause abortions by administering medicine if they became pregnant.

[citation needed] In July 1842, he wrote a series of letters to The Sangamo Journal, accusing Smith of conspiring to assassinate former Missouri Governor Boggs.

[7] In late 1842, Bennett published History of the Saints: Or, An Exposé of Joe Smith and Mormonism,[8] accusing Smith and his church of crimes including treason, conspiracy to commit murder, prostitution, and adultery, with a preface that begins: "I have been induced to prepare and publish the following work by a desire to expose the enormous iniquities which have perpetrated by one of the grosses and most infamous impostors that ever appeared upon the face of the earth, and by many of his minions, under the name and garb of Religion, and professedly by the direct will and command of Almighty God.

"[9] Through his newspaper writings and book, Bennett appeared to encourage Missouri's June 1843 attempt to extradite Smith to stand trial for treason.

The term was occasionally used by Mormon leaders such as Brigham Young, who spoke of the shock he received when introduced by Smith to "the spiritual wife doctrine", referring to "plural marriage".

The term "spiritual wifery", with its mixed connotations of polygyny and promiscuity, was frequently used in the national dialogue and in activism against Mormon polygamy.

[citation needed] Bennett's troubled relationship with the Mormons has overshadowed his other notable activities, including commanding a company for the Union in the American Civil War.

Beginning in 1835, Bennett was an early champion of the health benefits of the tomato; a pioneer in the use of chloroform as an anesthetic, publishing his findings in 1848; and the creator of several breeds of chicken, including Plymouth Rock fowl, which he exhibited in Boston in 1849.