William Crookes was born in London in 1832, the eldest of eight surviving children (eight others died young) of Joseph Crookes (1792–1889), a wealthy tailor and real estate investor of north-country origin, and his second wife, Mary (née Scott; 1806–1884). Joseph Crookes's father, William (1734-–814), was also a tailor, and his grandfather, John Crookes (b. 1660), had been Mayor of Hartlepool, County Durham on three occasions.
Joseph Crookes had had five children with his first wife; two sons from that marriage, Joseph and Alfred, took over the tailoring business, leaving William free to choose his own path. In 1848, at age 16, Crookes entered the Royal College of Chemistry (now the Imperial College chemistry department) to study organic chemistry. Crookes lived with his parents about three miles from the College in Oxford Street. His fathers shop was about half a mile away. Crookes paid £25 for his first years tuition and had to provide his own apparatus and some of the more expensive chemicals.  At the end of his first year, Crookes won the Ashburton scholarship which covered his second year's tuition. At the end of his second year, Crookes became a junior assistant to August Wilhelm von Hofmann, doing laboratory demonstrations and helping with research and commercial analysis.  In October 1851, Crookes was promoted to senior assistant, a position he held until 1854.
Although Crookes revered Hofmann, he did not share his primary interest in organic chemistry. 
One of Crookes's students was the Reverend John Barlow, Secretary of the Royal Institution, who chose to take a course in analytical chemistry. Through Barlow, Crookes met scientists such as George Gabriel Stokes and Michael Faraday.
Such friends reinforced Crookes's interest in optical physics which was respected by Hofmann.
By 1851, Crookes's interest in photography and optics caused his father to build him a laboratory in the garden at home for his research.
When Crookes embarked upon original work, it wasn't in organic chemistry, but rather into new compounds of selenium. These were the subject of his first published papers, in 1851. He worked with Manuel Johnson at the Radcliffe Observatory in Oxford in 1854, where he adapted the recent innovation of wax paper photography to machines built by Francis Ronalds to continuously record meteorological parameters. In 1855 he was appointed lecturer in chemistry at the Chester Diocesan Training College.
In April 1856 Crookes married Ellen, daughter of William Humphrey of Darlington. Since staff at Chester were required to be bachelors, he had to resign his position. Williams father, Joseph Crookes, gave the couple a house at 15 Stanley Street, Brompton. Ellens mother, Mrs. Humphrey, lived with them for the rest of her life, nearly forty years.  A devoted couple, William and Ellen Crookes had six sons and three daughters.  Their first child, Alice Mary (born 1857, later Mrs. Cowland) remained unmarried for forty years, living with her parents and working as an assistant to her father. Two of Crookes's sons became engineers, and two lawyers.
Married and living in London, Crookes sought to support his new family through independent work as a photographic chemist. In 1859, he founded the Chemical News, a science magazine which he edited for many years and conducted on much less formal lines than was usual for the journals of scientific societies. Between 1864 and 1869, he was also involved with the Quarterly Journal of Science. At various times he edited the Journal of the Photographic Society and the Photographic News.