As a boy, Leonardo was apparently an avid artist. His father, Ser Piero, must have recognized his talents, for he soon apprenticed him to a studio. Since many career paths were closed to the illegitimate child, perhaps art was an obvious choice for such a skilled child: an eventual career as a court artist was the most honorable career a bastard could hope for. Because many of Ser Piero's clients belonged to the clergy, he probably had a good idea of the art market; he took his boy to one of the two most respected workshops in the city, that of Andrea del Verrocchio.

Verrocchio had studied under the great artist Donatello, and he served as official sculptor to the ruling Medici family. He was not only a skilled artist but a skilled teacher, and Leonardo received training in all artistic genres, except for large wall murals and frescoes. Leonardo first went to study under Verrocchio in 1467, when he was fifteen. At first, like any apprentice, he would have had to perform simple chores, almost as a servant would. Later he would have learned to prepare pigments and canvases; then he would have drawn studies of Verrocchio's works and other models.

During Leonardo's youth, Florence was going through a golden age. The boy probably spent a great deal of time in society. Lorenzo de Medici, ruler of Florence, liked to hold frequent public festivals, and Leonardo no doubt attended these, delightedly beholding their extravagant splendor.

Leonardo's earliest known drawing dates from a feast day. It is a landscape; he must have been wandering the countryside while the city was in noisy celebration. The picture stands out in its attention to geologic and botanic detail.

Much of Leonardo's other early work is collaborative in nature: during the Renaissance, paintings were usually done by groups of artists, directed by a master. Leonardo's first known contribution to one his master's works was in Verrocchio's Baptism of Christ. Almost all critics agree that Leonardo painted the leftmost angel. Its face and hair have a light, graceful quality unlike the other figures in the painting. Leonardo was probably also responsible for the background. According to Vasari, Leonardo's first biographer, Verrocchio was so impressed with his pupil's work on the angel that he grew ashamed of his own talents, and swore never to paint again. And indeed, Verrocchio soon abandoned paint as a medium; however, a more likely explanation is that he simply decided that Leonardo was good enough to take over most of the workshop's painting so that Verrocchio could focus on his sculpture works, which he had always preferred and excelled in. Soon enough, Leonardo was probably doing works of his own, probably including the Annunciation.

Many critics maintain that Leonardo was responsible for the background vista in The Annunciation and no more. Others suggest that he also painted the painting's angel; and indeed the sleeve of the angel matches some sketches from one of Leonardo's notebooks. The wings are oddly realistic; and such eccentricity is easily attributed to a genius like Leonardo, especially given his interest in birds. However, some point out that the head of the angel is too flat to be the work of Leonardo. Whether or not Leonardo was very involved in the painting's actual execution, he most likely was responsible for the design of the picture; he could have sketched the composition and let others do the painting. Although the angel fills a much greater space than the Virgin, the painting still contains a unity characteristic of a great artist like Leonardo. The angel is moving towards the Virgin, and she is responding. The four pine trees in the background form a unity with the building.

Leonardo's career had a strong start. His master was renowned throughout Florence and had in the Medici family the greatest patrons of the Renaissance. From Verrocchio, Leonardo acquired a tendency to paint curly bunched hair, to create triangular compositions, and to paint girlish, stylized faces. As he matured, many of these learned habits wore off and Leonardo's own style matured. Even while he was an apprentice, his own style showed through. An x-ray of Verrocchio's Baptism reveals that Leonardo's brushstrokes are much lighter than those of his master, and it is obvious to the naked eye that he was the greater talent.

There is no indication of any conflict between the methodical master Verrocchio and his genius dilettante pupil. Despite Leonardo's inability to finish many of his projects, he seems to have been capable of great discipline, as evidenced by his ability to teach himself Latin and to cut open cadavers to learn about anatomy. He was probably a good pupil.

As a boy from the countryside, Leonardo must have been in awe of the city of Florence. While his artistic talents probably matched those of any of his contemporaries, he was not as sophisticated, and his love of luxury and fine clothes later in life is perhaps a result of the envy he felt as a Florentine youth of small means.