In 1498 Machiavelli was made the head of the second chancery. This is an office of archives for public or ecclesiastic records.
Machiavelli was the secretary of a council that held diplomatic negotiations in the state.
This included missions to the French king (1504, 1510-11), the Holy See (1506), and the German emperor (1507-8). It was during these diplomatic missions that Machiavelli could study political tactics.
For example, in 1504 he spend 5 months in France and studied how a strong nation is united by the ruling of one prince.
In 1502 he saw how Cesare Borgia revenged his rebellious captians at Sinigaglia.
When Piero Soderini became chief magistrate, Machiavelli worked for him. Machiavelli persuaded him to pass a law in 1505 to have a small army of Italian soldiers.
In 1509 Machiavelli led this small army to help free Pisa. They succeeded.
Machiavelli was suspected as a conspirator involved in the downfall of Gonfaloniere Soderini. He was arrested and later tortured for this - he continued saying that he was innocent. Later, when he was released from prison, he was forced to retire from public life. He was banished to his country estate at San Casciano. This is where he wrote The Prince, Discourses and two plays, amongst some other minor works as well.
From 1521 till 1525 he was employed as a historiographer.