Newton was born in Wapping, a district in London near the Thames. His father was a shipping merchant who was brought up as a Catholic but had Protestant sympathies, and his mother was a devout Independent, unaffiliated with the Anglican Church. She had intended Newton to become a clergyman, but she died of tuberculosis when he was six years old. For the next few years, while his father was at sea Newton was raised by his emotionally distant stepmother. He was also sent to boarding school, where he was mistreated. At the age of eleven, he joined his father on a ship as an apprentice; his seagoing career would be marked by headstrong disobedience.

As a youth, Newton began a pattern of coming very close to death, examining his relationship with God, then relapsing into bad habits. As a sailor, he denounced his faith after being influenced by a shipmate who discussed with him Characteristics of Men, Manners, Opinions, Times, a book by the Third Earl of Shaftesbury. In a series of letters Newton later wrote, "Like an unwary sailor who quits his port just before a rising storm, I renounced the hopes and comforts of the Gospel at the very time when every other comfort was about to fail me." His disobedience caused him to be pressed into the Royal Navy, and he took advantage of opportunities to overstay his leave.

He deserted the navy to visit Mary "Polly" Catlett, a family friend with whom he had fallen in love. After enduring humiliation for deserting, he was traded as crew to a slave ship.

Newton often openly mocked the captain by creating obscene poems and songs about him, which became so popular that the crew began to join in. His disagreements with several colleagues resulted in his being starved almost to death, imprisoned while at sea, and chained like the slaves they carried. He was himself enslaved and forced to work on a plantation in the British colony Sierra Leone near the Sherbro River. After several months he came to think of Sierra Leone as his home, but his father intervened after Newton sent him a letter describing his circumstances, and crew from another ship happened to find him. Newton claimed the only reason he left the colony was because of Polly.

While aboard the ship Greyhound, Newton gained notoriety as being one of the most profane men the captain had ever met. In a culture where sailors habitually swore, Newton was admonished several times for not only using the worst words the captain had ever heard, but creating new ones to exceed the limits of verbal debauchery. In March 1748, while the Greyhound was in the North Atlantic, a violent storm came upon the ship that was so rough it swept overboard a crew member who was standing where Newton had been moments before. After hours of the crew emptying water from the ship and expecting to be capsized, Newton and another mate tied themselves to the ship's pump to keep from being washed overboard, working for several hours. After proposing the measure to the captain, Newton had turned and said, "If this will not do, then Lord have mercy upon us!" Newton rested briefly before returning to the deck to steer for the next eleven hours. During his time at the wheel, he pondered his divine challenge.

About two weeks later, the battered ship and starving crew landed in Lough Swilly, Ireland. For several weeks before the storm, Newton had been reading The Christian's Pattern, a summary of the 15th-century The Imitation of Christ by Thomas à Kempis. The memory of his own "Lord have mercy upon us!" uttered during a moment of desperation in the storm did not leave him; he began to ask if he was worthy of God's mercy or in any way redeemable. Not only had he neglected his faith but directly opposed it, mocking others who showed theirs, deriding and denouncing God as a myth. He came to believe that God had sent him a profound message and had begun to work through him.

Newton's conversion was not immediate, but he contacted Polly's family and announced his intention to marry her. Her parents were hesitant as he was known to be unreliable and impetuous. They knew he was profane too but allowed him to write to Polly, and he set to begin to submit to authority for her sake. He sought a place on a slave ship bound for Africa, and Newton and his crewmates participated in most of the same activities he had written about before; the only immorality from which he was able to free himself was profanity. After a severe illness his resolve was renewed, yet he retained the same attitude towards slavery as was held by his contemporaries. Newton continued in the slave trade through several voyages where he sailed the coasts of Africa, now as a captain, and procured slaves being offered for sale in larger ports, transporting them to North America.

In between voyages, he married Polly in 1750, and he found it more difficult to leave her at the beginning of each trip. After three shipping voyages in the slave trade, Newton was promised a position as ship's captain with cargo unrelated to slavery. But at the age of thirty, he collapsed and never sailed again

Working as a customs agent in Liverpool starting in 1756, Newton began to teach himself Latin, Greek, and theology. He and Polly immersed themselves in the church community, and Newton's passion was so impressive that his friends suggested he become a priest in the Church of England. He was turned down by John Gilbert, Archbishop of York, in 1758, ostensibly for having no university degree, although the more likely reasons were his leanings toward evangelism and tendency to socialize with Methodists. Newton continued his devotions, and after being encouraged by a friend, he wrote about his experiences in the slave trade and his conversion. William Legge, 2nd Earl of Dartmouth, impressed with his story, sponsored Newton for ordination by John Green, Bishop of Lincoln, and offered him the curacy of Olney, Buckinghamshire, in 1764.

Olney was a village of about 2,500 residents whose main industry was making lace by hand. The people were mostly illiterate and many of them were poor. Newton's preaching was unique in that he shared many of his own experiences from the pulpit; many clergy preached from a distance, not admitting any intimacy with temptation or sin. He was involved in his parishioners' lives and was much loved, although his writing and delivery were sometimes unpolished. But his devotion and conviction were apparent and forceful, and he often said his mission was to "break a hard heart and to heal a broken heart". He struck a friendship with William Cowper, a gifted writer who had failed at a career in law and suffered bouts of insanity, attempting suicide several times. Cowper enjoyed Olney – and Newton's company; he was also new to Olney and had gone through a spiritual conversion similar to Newton's. Together, their effect on the local congregation was impressive. In 1768, they found it necessary to start a weekly prayer meeting to meet the needs of an increasing number of parishioners. They also began writing lessons for children.

Partly from Cowper's literary influence, and partly because learned vicars were expected to write verses, Newton began to try his hand at hymns, which had become popular through the language, made plain for common people to understand. Several prolific hymn writers were at their most productive in the 18th century, including Isaac Watts – whose hymns Newton had grown up hearing – and Charles Wesley, with whom Newton was familiar. Wesley's brother John, the eventual founder of the Methodist Church, had encouraged Newton to go into the clergy. Watts was a pioneer in English hymn writing, basing his work after the Psalms.

Newton and Cowper attempted to present a poem or hymn for each prayer meeting. The lyrics to "Amazing Grace" were written in late 1772 and probably used in a prayer meeting for the first time on 1 January 1773. A collection of the poems Newton and Cowper had written for use in services at Olney was bound and published anonymously in 1779 under the title Olney Hymns. Newton contributed 280 of the 348 texts in Olney Hymns; "1 Chronicles 17:16–17, Faith's Review and Expectation" was the title of the poem with the first line "Amazing grace! (how sweet the sound)".

When originally used in Olney, it is unknown what music, if any, accompanied the verses written by John Newton. Contemporary hymnbooks did not contain music and were simply small books of religious poetry. The first known instance of Newton's lines joined to music was in A Companion to the Countess of Huntingdon's Hymns (London, 1808), where it is set to the tune "Hephzibah" by English composer John Husband.

"Amazing Grace", with the words written by Newton and joined with "New Britain", the melody most currently associated with it, appeared for the first time in Walker's shape note tunebook Southern Harmony in 1847.

Another verse was first recorded in Harriet Beecher Stowe's immensely influential 1852 anti-slavery novel Uncle Tom's Cabin. Three verses were emblematically sung by Tom in his hour of deepest crisis. He sings the sixth and fifth verses in that order, and Stowe included another verse, not written by Newton, that had been passed down orally in African-American communities for at least 50 years. It was one of between 50 and 70 verses of a song titled "Jerusalem, My Happy Home", which was first published in a 1790 book called A Collection of Sacred Ballads:

When we've been there ten thousand years,
Bright shining as the sun,
We've no less days to sing God's praise,
Than when we first begun.

Newton later took over the church of St. Mary Woolnoth in London. Throughout his life he fearlessly dwelt on both his sins and his love for Mary, who predeceased him in 1790. He died on December 21, 1807, and they were buried together at St. Mary Woolnoth.

Lyrics