Alessandro was the son of Duke Ottavio Farnese of Parma and Margaret, the illegitimate daughter of the Habsburg Emperor Charles V. He had a twin brother, Charles, who only lived one month. His mother was the half - sister of Philip II of Spain and John of Austria. He led a significant military and diplomatic career in the service of Spain under the service of his uncle the King. He fought in the Battle of Lepanto (1571) and then in the Netherlands against the rebels.

He accompanied his mother to Brussels when she was appointed Governor of the Netherlands. In 1565 his marriage with Maria of Portugal was celebrated in Brussels with great splendour. Alexander Farnese had been brought up in Spain with his cousin, the ill - fated Don Carlos, and Don John, both of whom were about the same age as himself, and after his marriage he took up his residence at once in the court of Madrid.

It was seven years, however, before he had again the opportunity to display his great military talents. In the meantime the provinces of the Netherlands had revolted against Spanish rule. Don John, who had been sent as governor - general to restore order, found difficulties in the opposition from William the Silent, who had succeeded in uniting all the provinces in common resistance to King Philip II. In the autumn of 1577, Farnese was sent to join Don John at the head of reinforcements, and it was his able strategy and prompt decision at a critical moment that won the Battle of Gembloux in 1578. Shortly afterwards Don John, whose health had broken down, died. Farnese was appointed to take his place. He was confronted with important difficulties, but he proved himself more than equal to the task.

Perceiving that there were divisions in the ranks of his opponents between Catholic and Protestant, Fleming and Walloon, he skilfully set to work by persuasion, successfully taking advantage and fomenting the growing discord, and bring back the Walloon provinces' allegiance to the king. By the treaty of Arras, January 1579, he was able to secure the support of the 'Malcontents', as the Catholic nobles of the south were styled, to the royal cause. The reply to the treaty of Arras was the Union of Utrecht, concluded a few weeks later between the seven northern provinces, who abjured the sovereignty of King Philip and bound themselves to use all their resources to maintain their independence of Spanish rule. As soon as he had obtained a secure basis of operations in Hainaut and Artois, Farnese set himself in earnest to the task of reconquering Brabant and Flanders by force of arms. Town after town fell into his power. Tournai, Maastricht, Breda, Bruges and Ghent opened their gates.

He finally laid siege to the great seaport of Antwerp. The town was open to the sea, strongly fortified, and defended with resolute determination and courage by the citizens. They were led by the famous Marnix van St. Aldegonde and assisted by an ingenious Italian engineer named Gianibelli. The siege began in 1584 and called forth all of Farnese's military genius. He cut off all access to Antwerp from the sea by constructing a bridge of boats across the Scheldt from Calloo to Oordam, in spite of the desperate efforts of the besieged townspeople to prevent its completion. The terms offered included the clause that all Protestants had to leave the city within two years. This relatively disciplined capture should not be confused with the bloody events of the Spanish Fury on 4 November 1576. Farnese was clearly avoiding the mistakes of his predecessor Don Luis de Requesens, although fear of a repeat of Spanish atrocities could have been a factor in the fleeing of 60,000 Antwerp citizens (60% of the pre - siege population). With the Fall of Antwerp, and with Mechelen and Brussels already in the hands of Farnese, the whole of the southern Netherlands was once more placed under the authority of Philip. At one stage Holland and Zeeland, whose geographical position made them unassailable except by water, were hard pressed to retain territory. The poorly supplied English forces sent by Elizabeth I were duly defeated by the Duke.

In 1586, Alexander Farnese became Duke of Parma through the death of his father. He applied for leave to visit his paternal territory, but Philip would not permit him. He could not replace him in the Netherlands. However, while retaining him in his command at the head of a formidable army, the king would not give his sanction to his great general's desire to use it for the conquest of England. Farnese at first believed it possible to successfully invade England with a force of 30,000 troops, without significant naval protection, relying mainly on the hope of a native Catholic insurrection. Philip overruled him, and began the work that led to the Spanish Armada. As part of the general campaign preparations, Farnese moved against Ostend and Sluis. Sluis was taken in August 1587. The Armada reached the area a year later. After its defeat, Farnese broke up his camp in Dunkirk in September.

Farnese was to have turned his attention back to the northern Netherlands, where the Dutch had regrouped, but on 1–2 August 1589, Henry III of France was assassinated, and Farnese was ordered into France, in support of the Catholic opposition to Henry IV of France. This enabled the Dutch rebels to turn the tide in favour of the Dutch Revolt, which had been in ever deeper trouble since 1576. In September 1590 he moved to relieve Paris from the lengthy siege it had been placed under by Huguenots and Royalists loyal to Henry IV.

On 20 April 1592 he repeated the same deed at Rouen, but was subsequently wounded in the hand. His health quickly declining, Farnese called his son Ranuccio to command his troops. Returned to the Flanders, he was however removed from the position of governor by the Spanish court. He died in Arras on 3 December 1592, aged 47. Farnese became Duke of Parma and Piacenza in 1586, but never ruled, naming his son Ranuccio as regent.