She was seven years younger than her cousin, Queen Elizabeth I; as well, she was more feminine, voluptuous and alluring. However, as a relative of the Queen she had been a Maid of Honour and, 
during this time, there may have been an 'understanding' with Robert Dudley, Earl of Leicester.

In the period of 1560 to 1565 she married Walter Devereux, 1st Earl of Essex, and they became the parents of two sons and two daughters. After her children were born, she started a more passionate
liason with the Earl of Leicester. Leicester's objective had been to make Queen Elizabeth jealous as well as pay her back for the favours paid by the Queen to Thomas Heneage, a gentleman of the Privy Chamber, who was presentable enough but hardly of sufficient distinction and charm to account for Elizabeth's surprise attentions. Leicester had been advised by Sir Nicholas Throckmorton "make love to another lady, and discover how the Queen's Majesty takes it". The liason between Leicester and Lettice ended abruptly when the news of it reached Elizabeth. In between bouts of tears and bursts of wrath, the Queen sent Lettice from the Court and subjected Leicester to bitter scoldings and reproaches, accompanied by public warnings that "she could lower him just as easily as she had raised him". Meanwhile Elizabeth continued to pay attention to Thomas Heneage. Leicester then withdrew from Court but soon was restored to royal favour.

Several years later she met Leicester at Kenilworth and the flame kindled in their earlier relations not only broke out afresh but with far greater ardour. However, Lettice's husband, the Earl of Essex, had
been suspicious over his wife's flirtation with Leicester ten years before, and particularly over the later arrival of a son, Robert, who could have been Leicester's. In November 1575 Essex, who was in
Ireland, heard about Lettice being ill and hastened to England, declaring his intention of staying there for good. This presence held Leicester's ardour in check.

However, Leicester, one of the most influential members of the Privy Council, pressed that Essex should be sent back to Ireland to complete his mission. Despite the protests of the Earl of Essex, he
was ordered to go and in August set out for Dublin. Within a few days he fell ill with severe abdominal pains and died 22 September 1576, at the age of thirty-six, after vowing that he had been poisoned. A
post-mortem was held and death declared due to dysentery following the drinking of impure water.

Lettice and Leicester entered into a secret betrothal and she was discreetly installed at the White House at Wanstead, the manor and estate Leicester had recently acquired, six miles to the north-east of
London. Soon Lettice was with child and her father, Sir Francis Knollys, close kinsman and trusted servant of the Queen, insisted on a formal and binding ceremony of marriage. In September 1578, at Wanstead, they were wed. There was little that was secret about this ceremony; it was merely not public. Everyone knew of it save the Queen, for none had the hardihood to tell her. Leicester had opposed plans for the Queen to marry the French Duke of Alencon. Then to London came Monsieur Jean de Simier, envoy of Alencon. When an attempt was made to kill Simier, he chiefly suspected
Leicester and, in revenge, while the Court was at Greenwich, he revealed to the Queen the secret of the marriage.

Elizabeth went into a frantic rage and, in her passion, would have sent Leicester to the Tower but his magnanimous enemy, the Earl of Sussex, fearing a public scandal that would follow, persuaded her
to be content with confining him in the Miraflore Tower in Greenwich Park. Then Leicester's previous marriage to the very much alive Douglas Howard was revealed to the Queen. Douglas, herself remarried
and fearing Leicester, denied there had been a marriage. This disappointed Elizabeth as gladly she would have voided the marriage of Lettice and Leicester.

Then, in 1579, Lettice gave birth to a son, Robert. When this child was two years old, he was formally betrothed to Arabella Stuart, a first cousin of James VI, King of Scots, and herself in line to the
English throne. This was never revealed to Queen Elizabeth but what was revealed were the attempts to marry Lettice's daughter, Dorothy, to the young King of Scots, James VI. However, Dorothy eloped with Sir Thomas Perrot.

Some years later, in 1583, the Queen appointed Douglas's husband, Edward Stafford, as her ambassador to the King of France. In due course Douglas would regain her standing with the Queen but
Elizabeth remained implacable towards Lettice. As for Leicester, Elizabeth soon forgave him and allowed him to return to his former place and influence. However, on 19 July 1584, the son of Lettice and Leicester died. In 1584 Lettice's son, Robert Devereux, 2nd Earl of Essex, was introduced at Court by Leicester. Within three years he became a regular attender at Court and was spending a great deal of his time with the Queen.

In the years that followed, Leicester suspected that Lettice was unfaithful to him and, on 4 September 1588 at Cornbury, the Earl of Leicester died. Leicester had owed the Treasury some 70,000 pounds
and, once the Queen was over her initial grief, ensured that she obtained 20,000 pounds raised by the sale of effects. Rumours soon blamed Lettice for having poisoned Leicester, for she had, it was said, for some time been indulging in an affaire with his Gentleman of the Horse, the twenty-three year old Sir Christopher Blount, whom Leicester had himself knighted after the young soldier lost a hand 
fighting in the Netherlands war. Men of honour and sense dismissed these slanders, especially those who knew Leicester to be a sick man, until they were seemingly confirmed less than a year later when she and Blount, nearly a quarter of a century her junior, were married. Lettice's financial position was not improved by her extravagant new husband, on whom she doted, and who periodically sold fine pieces of her jewellery, as well as the leases and lordships of some of her possessions
.
Her son, Robert, became the favourite of Queen Elizabeth but paid the price for his treachery by being beheaded on 25 February 1601. Lettice's husband, Sir Christopher Blount, as he was involved in her son's plots, followed to the scaffold on 18 March 1601. Queen Elizabeth died on 24 March 1603 and Lettice, her cousin and rival, lived on for another thirty-one years to die on 25 December 1634.
 

Source: Leo van de Pas