Amasa Mason Lyman was the third son of Roswell Lyman and Martha Mason, and was born 30 March 1813 in Lyman Township, Grafton County, New Hampshire. When Amasa was age two, his father left to seek a farm in the West, but never returned and later was reported to have died. Amasa's mother married Isaiah Emerson and moved to the village of Holland, New Hampshire, leaving Amasa, then nine, with her aged parents. Within two years the grandparents died and Amasa stayed to live with his uncle Parley Mason. At age nineteen Amasa became a convert to the Church of Jesus Christ of Latter-day Saints, having been taught by Orson Pratt and Lyman E. Johnson. After his baptism in April 1832, Amasa was no longer welcome in the Mason home. With only $11.13 he made his way 700 miles to the Lyman Johnson farm. Amasa became a Mormon Church elder that August, and dedicated his life to the ministry.

Lyman was ordained an apostle at age 29 on 20 August 1842 by Brigham Young. He served twenty-five years as an apostle, including one and a half years in the church First Presidency as counselor to the prophet Joseph Smith. Amasa served with Zion's Camp, and was imprisoned with Joseph Smith in the Richmond jail. He was a regent of the University of Nauvoo, a justice of the peace, and a company captain in the first two pioneer treks to Utah. He helped to lead a detachment of the Mormon Battalion from Pueblo to the Great Salt Lake Valley, and helped to lay out the wards of Salt Lake City. He later served ten years in the Utah Territorial Legislature. He scouted the western and southern approaches to Utah and designed a defense against potential attack from enemies. He and Porter Rockwell evaluated the valley around Utah Lake for its first settlement.

Lyman filled a call to build and lead a Mormon community at San Bernardino with Charles C. Rich, and served as president of the California Mission (1853-54). He edited and managed the British periodical The Millennial Star, and served as European Mission President from 1860 to 1862, a position later filled by two of his sons, Platte D. Lyman (1898-1901) and Francis M. Lyman (1901-04).

Apostle Lyman helped form the Nauvoo Legion in Utah with Daniel H. Wells and Charles C. Rich. Amasa and his son Francis built and operated a flour mill and a sawmill, among the first of each in Utah. He worked diligently to help establish Utah communities in Fillmore, Parowan, Minersville, Farmington, and Salt Lake City. He was known as an entertainer and a gentleman, and as an expert in carpentry, iron work, fine mechanics, fruit and vegetable production, and cattle raising. Lyman was an avid reader and was well-informed on many subjects. Despite suffering from cancer, lingering internal injuries, other physical discomforts, persecution, and unjust property loss, he taught and preached encouragement, love, and kindness. When not out of the territory on assignments, he traveled around Utah with endless diligence assisting settlers.

Lyman had eight wives: Maria Louisa Tanner, Caroline Ely Partridge, Cornelia Eliza Leavitt, Dionita Walker, Eliza Maria Partridge, Paulina Eliza Phelps, Priscilla Turley, and Lydia Partridge. On 6 October 1867 Amasa M. Lyman was deprived of his apostleship for ambiguous preachings about the atonement of Jesus Christ; and he was excommunicated from the LDS Church on 12 May 1870. He died on 4 February 1877 at Fillmore, Utah. He was reinstated posthumously to church membership and apostleship on 12 January 1909.