Essex was born on 10 November 1565 at Netherwood near Bromyard, in Herefordshire, the son of Walter Devereux, 1st Earl of Essex, and Lettice Knollys. His maternal great-grandmother Mary Boleyn was a sister of Anne Boleyn, the mother of Queen Elizabeth I, making him a first-cousin-twice-removed of the Queen.

He was brought up on his father's estates at Chartley Castle, Staffordshire, and at Lamphey, Pembrokeshire, in Wales. His father died in 1576, and the new Earl of Essex became a ward of Lord Burghley. In 1577, he was admitted as a fellow-commoner at Trinity College, Cambridge; in 1579, he matriculated; and in 1581 he graduated as a Master of Arts.

On 21 September 1578, Essex's mother married Robert Dudley, Earl of Leicester, Elizabeth I's long-standing favourite and Robert Devereux's godfather.

Essex first came to court in 1584, and by 1587 had become a favourite of the Queen, who relished his lively mind and eloquence, as well as his skills as a showman and in courtly love. In June 1587 he replaced the Earl of Leicester as Master of the Horse. After Leicester's death in 1588, the Queen transferred the late Earl's royal monopoly on sweet wines to Essex, providing him with revenue from taxes. In 1593, he was made a member of her Privy Council.

In 1589, he took part in Francis Drake's English Armada, which sailed to Spain in an unsuccessful attempt to press home the English advantage following the defeat of the Spanish Armada, although the Queen had ordered him not to take part. In 1591, he was given command of a force sent to the assistance of King Henry IV of France. In 1596, he distinguished himself by the capture of Cádiz. During the Islands Voyage expedition to the Azores in 1597, with Walter Raleigh as his second-in-command, he defied the Queen's orders, pursuing the Spanish treasure fleet without first defeating the Spanish battle fleet.

Essex performed military Service under his stepfather in the Netherlands, before making an impact at court and winning the Queen's favour. In 1590, he married Frances Walsingham, daughter of Sir Francis Walsingham and widow of Sir Philip Sidney, by whom he was to have several children, three of whom survived into adulthood. Sidney, who was Leicester's nephew, had died in 1586 at the Battle of Zutphen in which Essex had also distinguished himself. In October 1591, Essex's mistress, Elizabeth Southwell, gave birth to a son who survived into adulthood.

Several of Essex's poems were set to music. English Composer John Dowland set a poem called "Can she excuse my wrongs with virtue's cloak?" in his 1597 publication First Booke of Songs: these lyrics have been attributed to Essex, largely on the basis of the dedication of "The Earl of Essex's Galliard", an instrumental version of the same song. Dowland also sets the opening verses of Essex's poem "The Passion of a Discontented Mind" ("From silent night") in his 1612 collection of songs. Orlando Gibbons set lines from the poem in the same year. Settings of Essex's poems "Change thy minde" (set by Richard Martin) and "To plead my faith" (set by Daniel Bacheler) are published in A Musicall Banquet (1610), a collection of songs edited by Robert Dowland.

He was the second Chancellor of Trinity College, Dublin, serving from 1598 to 1601.

Relying on his general warrant to return to England, given under the great seal, Essex sailed from Ireland on 24 September 1599, and reached London four days later. The Queen had expressly forbidden his return and was surprised when he presented himself in her bedchamber one morning at Nonsuch Palace, before she was properly wigged or gowned. On that day, the Privy Council met three times, and it seemed his disobedience might go unpunished, although the Queen did confine him to his rooms with the comment that "an unruly beast must be stopped of his provender."

Cecil kept up the pressure and, on 5 June 1600, Essex was tried before a commission of 18 men. He had to hear the charges and evidence on his knees. Essex was convicted, was deprived of public office, and was returned to virtual confinement.

Essex was found guilty and, on 25 February 1601, was beheaded on Tower Green, becoming the last person to be beheaded in the Tower of London. It was reported to have taken three strokes by the executioner Thomas Derrick to complete the beheading. Previously Thomas Derrick had been convicted of rape but was pardoned by the Earl of Essex himself (clearing him of the death penalty) on the condition that he became an executioner at Tyburn. At Sir Walter Raleigh's own execution on 29 October 1618, it was alleged that Raleigh had said to a co-conspirator, "Do not, as my Lord Essex did, take heed of a preacher. By his persuasion he confessed, and made himself guilty." In that same trial, Raleigh also denied that he had stood at a window during the execution of Essex's sentence, disdainfully puffing out tobacco smoke in sight of the condemned man. Essex at the end shocked many by denouncing his sister Penelope, Lady Rich as his co-conspirator: the Queen, who was determined to show as much clemency as possible, ignored the charge.

There is a widely repeated romantic legend about a ring given by Elizabeth to Essex. There is a possible reference to the legend by John Webster in his 1623 play The Devil's Law Case suggesting that it was known at this time, but the first printed version of it is in the 1695 romantic novel The Secret History of the most renowned Queen Elizabeth and the Earl of Essex, by a Person of Quality. The version given by David Hume in his History of England says that Elizabeth had given Essex a ring after the expedition to Cadiz that he should send to her if he was in trouble. After his trial he tried to send the ring to Elizabeth via the Countess of Nottingham, but the Countess kept the ring as her husband was an enemy of Essex, as a result of which Essex was executed. On her deathbed the Countess is said to have confessed this to Elizabeth, who angrily replied "May God forgive you, Madam, but I never can." The chapter house museum in Westminster Abbey has a gold ring which is claimed to be this one.

Some historians consider this story of the ring to be a myth, partly because there are no contemporary accounts of it. John Lingard in his history of England says the story appears to be a fiction, Lytton Strachey states "Such a narrative is appropriate enough to the place where it was first fully elaborated — a sentimental novelette; but it does not belong to history", and Alison Weir calls it a fabrication.

Robert Devereux's death and confession became the subject of two popular 17th-century Broadside Ballads, set to the English folk tunes Essex Last Goodnight and Welladay. Numerous ballads lamenting his death and praising his military feats were also published throughout the 17th century.