Ptolemy was born in Macedonia around 367 BCE and was known as the son of Lagus and Arsinoe. A later story made Philip II his father, but that claim cannot be treated as established parentage. He belonged to Alexander's circle and accompanied the expedition into Asia. In 330 BCE he became one of the king's personal bodyguards, an important court and military position rather than simply a soldier assigned to stand at a door. His surviving reputation comes partly from later writers who used his own account, so descriptions of his service also need to be read with attention to how he wished to be remembered.

When Alexander died at Babylon in 323 BCE, the empire's commanders faced a contested succession and a division of responsibilities among rival officers. Ptolemy obtained the satrapy of Egypt: a governorship within the empire, not yet an openly independent kingship. Egypt's revenues, agricultural resources and geographical position gave him a substantial base. He established his own authority there while other commanders attempted to preserve or control a wider imperial government. The resulting wars were not a simple contest in which every rival failed and Ptolemy alone survived; several lasting Hellenistic kingdoms emerged from the same struggles.

Alexander's burial became one of those struggles. The official Arrhidaeus supervised the elaborate funeral carriage, and Ptolemy secured the transfer of the body to Egypt around 321 BCE. Ancient accounts differ over the intended destination and the sequence of burials. Memphis appears as an initial burial place in one important tradition, while Alexander's tomb was later established at Alexandria; Pausanias attributes the transfer from Memphis to Ptolemy II. The evidence does not justify presenting Ptolemy I as certainly responsible for every stage. What is clear is that possession of the body gave the Egyptian court a powerful connection with the conqueror's memory.

Perdiccas, the empire's regent, invaded Egypt in 321/320 BCE. Diodorus describes fighting around fortified positions, disastrous attempts to cross the Nile and mounting hostility within the invading army. Perdiccas was killed by his own officers. Ptolemy retained Egypt, but the conflict involved real fighting and losses, not an effortless victory achieved without a battle. Further wars drew him into Syria, Cyprus and the Aegean. At Gaza in 312 BCE he and Seleucus defeated Demetrius, while the naval battle off Salamis in Cyprus in 306 BCE brought Ptolemy a serious defeat and the loss of the island.

The successors gradually abandoned the appearance of governing for Alexander's family. Ptolemy adopted the royal title in 305/304 BCE, around the same period as several of his rivals. During Demetrius's siege of Rhodes in 305-304 BCE, he supported the island with supplies and assistance. Ancient tradition connects the grateful Rhodians with his title Soter, although the precise development and official use of that epithet are more complicated than a single uncontested naming ceremony. Royal honours, military alliances and claims of protection were closely connected in the political culture of the period.

Alexandria, founded by Alexander in 331 BCE, became the centre of Ptolemaic royal life. Ptolemy I is traditionally associated with the beginnings of the Mouseion and royal library, but their foundation history cannot be reconstructed in every detail. Patronage continued under Ptolemy II, and the Pharos lighthouse was completed in the second king's reign. These institutions were the work of an early dynasty and its scholars, administrators and builders, not the finished creations of one founder. Ptolemy also supported Egyptian temples and promoted the cult of Serapis, whose identity drew upon existing Egyptian religious traditions rather than a god invented entirely from nothing.

The Satrap Stela of 311 BCE helps show how Ptolemy's position developed. It records benefits for the gods and temples of Buto while the formal royal dating still names Alexander IV. The document therefore belongs to Ptolemy's period as satrap; its Egyptian royal imagery should not be taken as proof that he had already adopted the kingly title in that year. Later Ptolemaic rule combined a Macedonian royal court with Egyptian offices, temple relationships and forms of kingship. Those connections involved negotiation and administration as well as the images presented on monuments.

Succession within Ptolemy's own family also required a choice. His children by Eurydice and Berenice I belonged to different branches of the royal household. He selected his son by Berenice, the future Ptolemy II Philadelphus, and associated him with kingship in 285/284 BCE. The elder Ptolemy died in 283/282 BCE, with the precise dating varying between reconstructions. His history of Alexander's campaigns did not survive as an independent work, but Arrian later used it as a major source. That makes Ptolemy both a participant in the events and a contributor to their later interpretation; it does not make his account free of self-interest.