In her early youth, Lettice was educated by the Reading scholar, Jocelyn Palmer, who was soon to become a Protestant martyr. In her late teens, she spent much time at her father's town-house in Reading, where, no doubt, she was nourished by her father's resolute faith that she clung to throughout her long life. References are not wanting in contemporary writings to show that the fatal gift of beauty had not been denied her and we soon find her as a maid of honour at court and amid the glittering throng surrounding Queen Elizabeth at the palaces of Whitehall, Greenwich and Windsor, "moving, doubtless, a foremost figure among the younger divinities of that heaven" and noting with interest, if not occasional alarm, the "states and stomachs of the Norreyses and the Knollyses" who were apparently always at loggerheads. It was probably under such conditions that she met her first husband, Walter Devereux, Viscount Hereford and, afterwards, Earl of Essex.

Essex had succeeded his father as second Viscount Hereford in September 1558, a few weeks before Elizabeth came to the throne. He was then a youth just entering his eighteenth year. Lettice and he were probably married around 1562, and five children soon joined the union: Penelope, Dorothy, Robert, Walter and Francis (who died young). The couple's favoured place at court underwent a minor blip in 1568, when Devereux was implicated in the plans for bringing about a marriage between Mary, Queen of Scots, and the Duke of Norfolk. The Queen was not amused and the couple were obliged to retire to the Viscount's Welsh homeland. Nothing is heard of them until the end of the following year, when Devereux was back, assisting in the break-up of the Northern Rebellion. About this time, he appears to have become particularly friendly with the Earl of Leicester and, possibly through the latter's influence, was raised further within the peerage to the Earldom of Essex. This was the zenith of his career. Very soon afterwards, he proposed that the Queen allow him to undertake the pacification and reduction of a certain disturbed district in the north of Ireland, on condition of being put in possession of half the lands he should recover from the rebels. This adventure was apparently suggested to him by Leicester who wanted him out of the way for, Fuller says, he "loved the Earl's nearest relation [meaning his wife] better than he loved the Earl himself." Lettice remained in London, at Durham House on the Strand, only a stone's throw from Leicester House. In the Summer of 1575, the two were both on progress with the Queen, when the Sheriff of Warwickshire refused to wear Leicester's livery at Kenilworth Castle because the latter - whom he called a whoremaster - had "private access to the Countess of Essex."

In December 1575, a Spaniard named de Guaras reported that there was a "great enmity between the Earl of Leicester and the Earl of Essex in consequence....of the fact that while Essex was in Ireland his wife had two children by Leicester." Other rumours indicated that the lady had had at least one abortion. It is therefore, perhaps, not surprising that, when Essex's venture turned out to be a complete failure, the financially embarrassed Earl resigned his command in Ireland but made no attempt to rejoin his wife and family after nearly two years' absence from them. Later, he appears to have come to England, but returned to Ireland in the Spring of 1576 with the comparatively insignificant title of Earl Marshal of Ireland. On 22nd September of that year, he died from a fatal attack of dysentery.

Almost ten years later, a book called "Leicester's Commonwealth," was published, which placed the responsibility for the Earl of Essex's death firmly at the hands of his wife and her lover, the Earl of Leicester. A poison was said to have been prepared by an Italian surgeon who had recently attached himself to Leicester. It was administered in a cup of wine at a merchant's house in Dublin and ended up killing a certain Mrs. Alice Draycot at the same time! This was not the first murder with which Leicester had been charged by popular opinion. Lord Robert Dudley, as he was originally called, was the fifth of the eight sons of John Dudley, Earl of Warwick, later Duke of Northumberland, who was beheaded on Tower Hill in 1553 after master-minding the accession to the throne of his daughter-in-law, the hapless Lady Jane Grey. Robert's dashing good looks had allowed him to rise quickly in the Queen's favour and she had showered him with lucrative appointments, gifts of land and the title of Earl of Leicester. It was widely rumoured that the monarch was in love with him and Dudley certainly did his best to realize his ambition to be King of England. Hence, he did not emerge from the death of his first wife, Amy Robsart, with an unblemished reputation.

Having finally abandoned all hopes of the Queen's hand, Leicester had turned to wooing the Countess of Essex, an enterprise which, as already noted, was provocative of no little scandal. Even Shakespeare appears to refer to their liaison - with an obscurity beyond most people's understanding - during Oberon's vision in the "Midsummer Night's Dream." In 1578, Lettice Knollys married the Earl of Leicester. The ceremony was first performed at Kenilworth, but Lettice's father, Sir Francis Knollys, who was well aware of Leicester's inconstancy, insisted that the union should be performed in his own presence, with some witnesses by, and a public notary. The ceremony was thus repeated, on 21st September 1578 at Wanstead in Essex, in the presence of Leicester's brother, the Earl of Warwick, Lord North, the lady's father and others. Lettice is said to have looked as if she were with child at the time. The couple's only son, Robert, Lord Denbigh - "The Noble Impe" - was certainly born some time during the following year; but he died at Wanstead on 19th July 1584. Lettice was back at Court in July 1579, with a new wardrobe that rivalled the Queen's, but the fact of her marriage was still carefully kept from Elizabeth, although very many courtiers were in on the secret. In August 1579, however, M. de Simier, the French Ambassador who was negotiating the Queen's marriage to the Duke of Alençon, suddenly broke the news to her. Elizabeth behaved as if she were heartbroken and, three days later, promised to accept Alençon on his own terms. She is said to have boxed the Countess' ears and banished her, commenting that as "but one sun lighted the sky," so she would "have but one Queen of England". She ordered Leicester to confine himself to Greenwich Palace and talked of sending him to the Tower, but Sussex advised her to be merciful. Leicester's friends declared that he voluntarily became a prisoner in his own chamber on the pretence of being unwell. In contrast to her husband, Lettice rebelled in style; and, in a series of attempts at being mistaken for her look-alike Royal cousin, she would ride through the streets of London in her carriage, with her ladies in coaches behind her. The ruse was usually successful. The couple's fall from grace was apparent to all and in the November, Leicester wrote a letter to Burghley, in which he compained, "I have lost both youth and liberty and all my fortune reposed in her."

During their married life, the fickle Queen periodically elevated and depressed Leicester by the award of honours and the denial of liberty. On 8th December 1585, Leicester was sent to the Low Countries and, on 25th January following, he made Governor-General of the Netherlands. Lettice proposed to join him there and set up a Court of her own; but Elizabeth discovered her plans and forbade her to leave the country. Soon after Leicester's return, an army was raised in contemplation of the Spanish invasion. Elizabeth intended him to be  her Lieutenant-General for England and Ireland but, deferring to the advice of Burghley and Hatton, the letters patent were never executed. In disgust, Leicester left the Court for Kenilworth Castle, but broke his journey at a house he had at Cornbury in Oxfordshire. Here, under somewhat mysterious circumstances, he was taken suddenly ill and died of "a continual fever, as 'twas said" on 4th September 1588.

Once again, rumour flew on the wings of the wind and the name of Lettice Knollys was on every tongue. Current gossip, at no time more venomous and widespread than in Elizabeth's day, immediately fastened upon her the guilt of Leicester's death by poison. It was remarked that Leicester died within seven or eight miles of Cumnor, the spot where Amy Robsart had met her death almost on the very day, twenty-eight years before; and it was bruited abroad that it was only justice that the wife of his youth should be avenged by the wife of his age. Ben Jonson tells the story that Leicester had given his wife "a bottle of liquor which he willed her to use in any faintness, which she, not knowing it was poison, gave him, and so he died". Bliss, in his notes to the 'Athenae Oxon,' was the first to print a contemporary narrative to the effect that the Countess had fallen in love with Christopher Blount, Leicester's Gentleman of the Horse; that Leicester had taken Blount on an expedition to Holland with the intention of killing him, but had failed in the endeavour; that the Countess, suspecting her husband's plot, gave him a poisonous cordial after a heavy meal while she was alone with him at Cornbury. Blount married the Countess after Leicester's death, and the narrator of the story gives as his authority William Haynes, Leicester's page and gentleman of the bedchamber, who saw the fatal cup handed to his master. But the story seems improbable in face of the post-mortem examination, which was stated to show no trace of poison.

Leicester was buried in the Beauchamp Chapel of the Collegiate Church in Warwick. The gorgeous funeral cost £4,000 and there, near the grave of their son,  Lord Denbigh, Lettice erected an elaborate altar-tomb, with a long Latin inscription to his memory. Dated at Middleburg, 1st August 1587, Leicester's will appointed Wanstead was for the Countess's Dower-House. She was his sole executrix, and proved the will two days after his death. His personal estate was valued at £29,820. The Countess, of course, resisted the efforts of Leicester's son by Lady Douglas Sheffield, Sir Robert Dudley - self-styled 'Duke of Northumberland' - to prove his legitimacy.

After marrying Sir Christopher Blount in July 1589 - another unpopular move with the Queen - Lettice lived a life of the disgraced, mostly at Drayton Bassett in Staffordshire. She remained on friendly terms, however, with her son by her first marriage, Robert, 2nd Earl of Essex, the Queen's new favourite and took some part in the education of her grandson, Robert (later 3rd Earl of Essex). In 1597, the Earl made a number of attempts to reconcile the two ladies. The Queen avoided two meetings before the two eventually came face to face, and Lettice was able to present her cousin with a jewel. A subsequent request for permission to return to Court was, however, refused and the Countess was, once more, forbidden access to the Monarch. After a quarrel with the Queen the following year, Essex was hiding at Wanstead in a strop when his mother advised him against imitating Leicester, who often feigned illness in the hope of winning the Queen over. Later, in 1599, Lettice was forbidden to see her son during his imprisonment at York House (Whitehall) but, upon his move to Essex House where she had been living, she was permitted a single interview. In February 1601, Essex  took the lead in a foolish plot against the Queen for which both he and Blount, his step-father, were executed on Tower Hill on 25th February and 17th March respectively. Lettice retired to Drayton Bassett in Staffordshire and died there, vigorous to the last, on 25th December 1634, aged 94. She was buried beside the Earl of Leicester at Warwick and some verses by her great grandson, Gervase Clifton, concerning her life, were painted on a tablet hung hear her tomb: "she was in her younger years matched with two great English peers; she that did supply the wars with thunder, and the court with stars".