From 1971 to 1974, Cannon and her husband served in the church's SwitzerlandMission, while he was the mission president. Upon their return to Utah in 1974, Cannon became the first counselor to Smith in the general presidency of the church's Relief Society. Cannon served in this capacity until 1978, when she was released so that she and her husband could become the first missionaries of the church to preach in "black Africa". They — along with Rendell and Rachel Mabey — preached in Nigeria and Ghana, baptized hundreds of converts, and established 27 branches of the LDS Church in Nigeria and Ghana. The first convert baptized in Nigeria was Anthony Obinna.
In the late 1980s, Cannon and her husband were the directors of the LDS Church's visitors' center in Nauvoo, Illinois. In 1989, they served as interim leaders of the church's GermanyHamburg Mission; during this time, the regular president of the mission was working on getting the church's missionaries admitted to East Germany.
Cannon was a member of the Mormon Tabernacle Choir for 18 years and during this time edited the choir's newsletter, Keeping Tab. Cannon was the editor or author of a number of books on LDS Church-related topics.
Janath R. Cannon (ed.) (1991). Nauvoo Panorama: Views of Nauvoo Before, During, and After Its Rise, Fall, and Restoration (Nauvoo, Ill.: Nauvoo Restoration Inc.)