Dudley was born around 1532, a younger son of the Duke of Northumberland, who was executed in 1553 for his part in the attempt to put Lady Jane Grey on the throne of England. (Lady Jane was married to Robert's elder brother, Guilford Dudley.) Robert Dudley was temporarily imprisoned, along with his father and brother, in the Tower of London, and it was here that he probably first came into contact with the then Princess Elizabeth, who had been sent there on the orders of her estranged elder sister, Queen Mary I of England. By this time he was already married, to Amy Robsart.
On Elizabeth's accession, Dudley was appointed Master of the Horse. Rumours about their relationship were rife, and when, in 1560, Dudley's wife, Amy, died after falling down a flight of stairs in mysterious circumstances, it was widely believed that he had arranged her murder in order to free himself to marry the queen. Some said that a secret marriage had taken place. Ironically, it would be Amy's death that put an end to any such ambitions Dudley may have had. Elizabeth, mindful of public opinion and also doubtful about the desirability of any marriage at all, never gave any cause to think that she intended to take the step of making her favourite into her husband. Historians today think Amy's murder, if it was a murder, was carried out either by someone who believed it would win them royal favour or, even more likely, by someone who wanted to prevent the queen from marrying Dudley. It seems impossible that Elizabeth could have been foolish enough to involve herself in such a crime, even if Dudley was.
(It has also been suggested that Amy was mortally ill with breast cancer at the time, which would have made killing her pointless. It also would give her a reason to kill herself, and makes a fatal accident plausible since advanced breast cancer can weaken the spine.)
In fact, in 1563, she put Dudley up as a candidate for marriage to the widowed Mary, Queen of Scots, whom she hoped to neutralise by marriage to a Protestant. The State Papers record how she hinted that this was to be a reward to Dudley, "whom, if it might lie in our power, we would make owner or heir of our own kingdom", for his loyal service. Mary, insulted by the idea of accepting Elizabeth's former lover, rejected him. In the following year, Elizabeth bestowed on him the earldom of Leicester.
Dudley was always a ladies' man. He married, secretly, in 1573, the widowed Lady Sheffield but later deserted her in favour of Lettice Knollys -- widow of Walter Devereux, 1st Earl of Essex, and Elizabeth's own cousin on her mother's side -- a marriage which offended the queen mightily. Eventually restored to her favour, Dudley was placed in command of the Dutch campaign of 1585, culminating in the Battle of Zutphen. Despite having shown himself a failure as a military leader, he was in command of the English land forces against the Spanish Armada of 1588. The Spanish never landed, and he died soon after near Oxford. By the time of his death, he was already losing his place as Elizabeth's favourite to his stepson, Robert Devereux, 2nd Earl of Essex.