Rockefeller was the second of six children and eldest son born in Richford, New York to con artist William Avery "Bill" Rockefeller (November 13, 1810 - May 11, 1906) and Eliza Davison (September 12, 1813 - March 28, 1889). His siblings were Lucy (1838-1878), William Jr. (1841-1922), Mary (1843-1925) and twins Franklin (Frank) (1845-1917) and Frances (1845-1847). His father was of English and German descent while his mother was of Scots-Irish descent. Bill was first a lumberman and then a traveling salesman who identified himself as a "botanic physician" and sold elixirs.

In 1859, Rockefeller went into the produce commission business with a partner, Maurice B. Clark, and they raised $4,000 ($108,948 in 2017 dollars) in capital. Rockefeller went steadily ahead in business from there, making money each year of his career. While his brother Frank fought in the Civil War, Rockefeller tended his business and hired substitute soldiers. He gave money to the Union cause, as did many rich Northerners who avoided combat. Rockefeller was an abolitionist who voted for President Abraham Lincoln and supported the then-new Republican Party. As he said, "God gave me money", and he did not apologize for it. He felt at ease and righteous following Methodist preacher John Wesley's dictum, "gain all you can, save all you can, and give all you can."  At that time, the Federal government was subsidizing oil prices, driving the price up from $.35 a barrel in 1862 to as high as $13.75. This created an oil-drilling glut, with thousands of speculators attempting to make their fortunes. Most failed, but those who struck oil did not even need to be efficient. They would blow holes in the ground and gather up the oil as they could, often leading to creeks and rivers flowing with wasted oil in the place of water.  In this environment of wasteful boom, the partners switched from foodstuffs to oil, building an oil refinery in 1863 in "The Flats", then Cleveland's burgeoning industrial area. The refinery was directly owned by Andrews, Clark & Company, which was composed of Clark & Rockefeller, chemist Samuel Andrews, and M. B. Clark's two brothers. The commercial oil business was then in its infancy. Whale oil had become too expensive for the masses, and a cheaper, general-purpose lighting fuel was needed.  While other refineries would keep the 60% of oil product that became kerosene, but dump the other 40% in rivers and massive sludge piles, Rockefeller remained as thrifty and efficient as ever, using the gasoline to fuel the refinery, and selling the rest as lubricating oil, petroleum jelly and paraffin wax, and other by-products. Tar was used for paving, naphtha shipped to gas plants. Likewise, Rockefeller's refineries hired their own plumbers, cutting the cost of pipe-laying in half. Barrels that cost $2.50 each ended up only $.96 when Rockefeller bought the wood and had them built for himself.  In February 1865, in what was later described by oil industry historian Daniel Yergin as a "critical" action, Rockefeller bought out the Clark brothers for $72,500 (equivalent to $1 million in 2017 dollars) at auction and established the firm of Rockefeller & Andrews. Rockefeller said, "It was the day that determined my career." He was well positioned to take advantage of postwar prosperity and the great expansion westward fostered by the growth of railroads and an oil-fueled economy. He borrowed heavily, reinvested profits, adapted rapidly to changing markets, and fielded observers to track the quickly expanding industry.

What happened when the War ended?
action, Rockefeller bought out the Clark brothers for $72,500 (equivalent to $1 million in 2017 dollars) at auction