Alexander was born in July 356 B.C., the sixth day of the Macedonian month Loïos, to King Philip II and his wife Myrtale (better known to us now by her adopted name, Olympias). Under Philip, Macedonia flourished and grew, while Olympias proved to be the most powerful of his wives. Before Philip's reign, other Greek nations looked down upon Macedonia as barbarian, with obsolete political institutions, coarse speech and manners, and little to offer in battle. Philip therefore began reforms and expansion that Alexander would bring to a peak for Macedonia.

Two marriages brought Philip no suitable male heir. His third wife would be the niece of King Arrybas of Epirus (and daughter of the deceased King Neoptolemus). Plutarch asserts that the two met and fell in love years before their union became politically convenient. In any case, Epirus's allegiance would be useful to Philip, and Arrybas gladly approved of their marriage. Just fifteen years later, Philip would drive Arrybas from the throne and replace him with a brother of Olympias.

Philip and Olympias were far from a happy couple. Philip went on to marry three more women, and he is said to have fathered several children by still other partners, while also enjoying the company of young boys–none of which was particularly unusual in his time. These exploits may have wounded Olympias's vanity, but they did not hold back her ambition. From the beginning she showed a forceful personality, though not even eighteen at the time of their marriage. Olympias was devoted to the orgiastic rites of Dionysus, and her eccentricity did not make her easier to get along with.

This disruptive family environment would have significant consequences for Alexander's development, though the extent of the troubles between mother and father is disputed. The mutual dislike of his parents was complicated by his father's absence–away on campaigns–during most of his earliest years.

Alexander's mother, therefore, was responsible for guiding his formative years. Her first priority was to instill in him a sense of destiny and the greatness to which he would ascend. She may also have tried to turn him against his father, especially criticizing Philip's moral shortcomings. This indoctrination likely contributed to the dislike that developed between father and son, while Alexander always held his mother in the deepest respect, despite knowledge of her less scrupulous actions. Moreover, the dynamics of these relationships likely contributed to the sexual reluctance or restraint apparent in Alexander's later years. On the other hand, however, Alexander did feel genuine admiration for his father, and in many ways he followed Philip's path as a military leader and king.

The details of Alexander's early life are difficult to confirm with direct evidence amid the many legends that surround his life. Most accounts do paint him as a precocious child, accustomed to association with great politicians, artists, and generals, from whom he quickly learned through imitation. Among other talents, such as archery and javelin, Alexander showed a particular aptitude for horsemanship.

One famous anecdote recalls the acquisition of Bucephalas, the prize horse of a certain breeder who came to sell the horse to Philip for a great sum. When the king's servants found the horse unmanageable, Philip sent the breeder away, only to be interrupted by his eight-year-old son, who complained that a great horse should not be lost simply because no one had the skill or courage to master him. Alexander then rose to the challenge and tamed the horse proudly in front of a speechless audience, leaving Philip likely filled with pride and perhaps a little resentment. The horse was bought for Alexander; Bucephalas went on to serve him in almost every significant battle until his death at the age of thirty, after his master's last major victory in India.

Another anecdote paints a more troubling picture of Alexander. Once, in offering a sacrifice, Alexander scooped up two whole handfuls of incense and tossed them into the altar-fire. His tutor, Leonidas, rebuked him, reportedly saying, "When you've conquered the spice-bearing regions, you can throw away all the incense you like. Till then, don't waste it." Years later, Alexander would capture Gaza, a major spice producer. Along with the usual gifts for his mother and sister, he included a consignment of eighteen tons of frankincense and myrrh for the old tutor Leonidas–the resale of which would have made him exorbitantly rich. This was delivered "in remembrance of the hope with which that teacher had inspired his boyhood," along with a warning to cease being stingy to the gods. On the one hand, Alexander's actions demonstrate considerable generosity, if in a mocking manner. However, this anecdote also reveals Alexander's capacity for holding grudges; he is known for never forgetting an injury. Though he waited with patience, he rarely failed in the end to carry out his vengeance.