Born in Florence, he was recognized by the majority of his contemporaries as the only son of Lorenzo II de' Medici (grandson of Lorenzo de' Medici "the Magnificent"). A few believed him to be in fact the illegitimate son of Giulio de' Medici (later Pope Clement VII), nephew of Lorenzo the Magnificent, but at the time that was a minority view.
Some historians (such as Christopher Hibbert) believe he had been born to a servant of African descent who was working in the Medici household, identified in documents as Simonetta da Collevecchio. The French author Jean Nestor, writing in the 1560s, reported that the claim of a Moorish slave origin was a false rumor first spread by Alessandro's exiled enemies in Naples. Alessandro's nickname "il Moro" ("the Moor") is said to derive from his features.
The Emblem of Alessandro de' Medici
, based on Dürer's Rhinoceros
, with the motto "Non buelvo sin vencer" (old Spanish for "I shall not return without victory").
(From Paolo Giovio's Dialogo dell'impresse militari et amorosi
When Emperor Charles V sacked Rome in 1527, the Florentines took advantage of the turmoil in Italy to reinstall the Florentine Republic. Both Alessandro and Ippolito de' Medici fled, along with the rest of the Medici and their main supporters, including Cardinal Silvio Passerini, the regent for Pope Clement VII (at that time the ruler of Florence), with the exception of the eight-year-old Catherine de' Medici, who was left behind. Michelangelo, then occupied in creating a funerary chapel for the Medici, initially took charge of building fortifications around Florence in support of the Republic; he later fled the city temporarily. Clement eventually made his peace with the Emperor, and with the support of Imperial troops, the Republic was overwhelmed after a lengthy siege, and the Medici were restored to power in the summer of 1530.
Clement assigned Florence to nineteen-year-old Alessandro, who had been made a duke, an appointment that was purchased from Charles. He arrived in Florence to take up his rule on 5 July 1531 and was made hereditary Duke of Florence 9 months later by the Emperor (as Tuscany lay outside the Papal States), thereby signalling the end of the Republic.
Alessandro's many enemies among the Florentine exiles declared that his rule was harsh, depraved and incompetent, an assessment debated by later historians. One relic of his rule sometimes pointed out as a symbol of Medici oppression is the massive Fortezza da Basso, a fortress that is today the largest historical monument of Florence. In 1535, the Florentine opposition sent his cousin Ippolito to appeal to Emperor Charles V against some actions of the duke, but Ippolito died en route; rumors were spread that he had been poisoned at Alessandro's orders.
In a late replay of the kind of medieval civil politics that had long revolved around pope and emperor, commune and lord, Emperor Charles V supported Alessandro against the republicans. In 1536, Charles V married his natural daughter Margaret of Austria to Alessandro. For his own inclinations, Alessandro seems to have remained faithful to one mistress, Taddea Malaspina, who bore his only children: Giulio de' Medici (c. 1533/37-1600), who also had illegitimate issue, and Giulia de' Medici.