The son of Nero Claudius Drusus, a popular and successful Roman general, and the younger Antonia, he was the nephew of the emperor Tiberius and a grandson of Livia Drusilla, the wife of the emperor Augustus. Ill health, unattractive appearance, clumsiness of manner, and coarseness of taste did not recommend him for a public life. The imperial family seems to have considered him something of an embarrassment, and he was long left to his own private studies and amusements. It was the historian Livy who recognized and encouraged his inclination for historical studies. Claudius wrote a pamphlet defending the republican politician and orator Cicero, who was executed by the triumvirs; and, having discovered that it was difficult to speak freely on the civil wars toward the end of the Roman Republic, he began a history of Rome with the principate of Augustus. He composed 20 books of Etruscan and 8 books of Carthaginian history, all in Greek; an autobiography; and a historical treatise on the Roman alphabet with suggestions for orthographical reform—which as emperor he later tried not very successfully to implement. He also wrote on dice playing, of which he was fond. All his works are lost, and their importance cannot be measured. The Etruscan history may have had original material: his first wife, Plautia Urgulanilla, had Etruscan blood, and her family was probably able to put Claudius in touch with authentic Etruscan traditions. After divorcing Urgulanilla, he in turn married Aelia Paetina, Valeria Messalina, who was his wife at his accession, and, finally, Agrippina the Younger. By his first three wives he had five children, of whom Drusus and Claudia died before he became emperor. As a young man Claudius was made a member of various religious colleges, but he became consul only under the reign of his older brother's son Gaius (Caligula) in 37. There was, however, little cordiality between the two.

Power came to Claudius unexpectedly after Gaius' murder on Jan. 24, 41, when he was discovered trembling in the palace by a soldier. The Praetorian Guards, the imperial household troops, made him emperor on January 25. By family tradition and antiquarian inclinations, Claudius was in sympathy with the senatorial aristocracy; but soldiers and courtiers were his real supporters, while freedmen and foreigners had been his friends in the days of neglect. Initially, the attitude of the Senate was at best ambiguous. In 42 many senators supported the ill-fated rebellion of the Governor of Dalmatia. Even later, several attempts on Claudius' life involved senators and knights. Though paying homage to the dignity of the Senate (to whose administration he returned the provinces of Macedonia and Achaea) and giving new opportunities to the knights, Claudius was ruthless and occasionally cruel in his dealings with individual members of both orders. From the very beginning he emphasized his friendship with the army and paid cash for his proclamation as emperor.

Claudius' decision to invade Britain (43) and his personal appearance at the climax of the expedition, the crossing of the Thames and the capture of Camulodunum (Colchester), were prompted by his need of popularity and glory. But concern with the anti-Roman influence of the Druid priesthood, which he tried to suppress in Gaul, and a general inclination toward expanding the frontiers were other reasons. Claudius planted a colony of veterans at Camulodunum and established client-kingdoms to protect the frontiers of the province; these were afterward a source of trouble, such as the revolt in 47 of Prasutagus, client-king of the Iceni, and later the general revolt instigated by his wife Boudicca (also called Boadicea). He also annexed Mauretania (41–42) in North Africa, of which he made two provinces (Caesariensis in the east and Tingitana in the west), Lycia in Asia Minor (43), and Thrace (46). Though he enlarged the kingdom of Herod Agrippa I, he later made Judaea a province on Agrippa's death in 44. In 49 he annexed Iturea (northeastern Palestine) to the province of Syria. He was careful not to involve the empire in major wars with the Germans and the Parthians (Parthia). Claudius supported Roman control of Armenia, but in 52 he preferred the collapse of the pro-Roman government to a war with Parthia, leaving a difficult situation to his successor.

In the civil administration, many measures demonstrate Claudius' enlightened policy. He improved in detail the judicial system, and, in his dealings with the provinces, he favoured a moderate extension of Roman citizenship by individual and collective grants: in Noricum, a district south of the Danube comprising what is now central Austria and parts of Bavaria, for instance, five communities became Roman municipalities. He encouraged urbanization and planted several colonies, for example, at Camulodunum and at Colonia Agrippinensis (modern Cologne) in Germany in 51. In his religious policy Claudius respected tradition; he revived old religious ceremonies, celebrated the festival of the Secular Games in 47 (three days and nights of games and sacrifice commemorating the 800th birthday of Rome), made himself a censor in 47, and extended in 49 the pomerium of Rome (i.e., the boundary of the area in which only Roman (Roman religion) gods could be worshipped and magistrates ruled with civil, not military, powers). He protected the haruspices (diviners) and probably Romanized the cult of the Phrygian deity Attis. According to the biographer Suetonius in Claudius, during a period of troubles Claudius expelled the Jews (Jew) from Rome for a short time; Christians may have been involved. Elsewhere he confirmed existing Jewish rights and privileges, and in Alexandria he tried to protect the Jews without provoking Egyptian nationalism. In a surviving letter addressed to the city of Alexandria, he asked Jews and non-Jews “to stop this destructive and obstinate mutual enmity.” Although personally disinclined to accept divine honours, he did not seriously oppose the current trend and had a temple erected to himself in Camulodunum. His public works include the reorganization of the grain supply of Rome and construction of a new harbour at Ostia, which was later improved by the emperor Trajan.

Claudius' general policy increased the control of the emperor over the treasury and the provincial administration and apparently gave jurisdiction in fiscal matters to his own governors in the senatorial provinces. He created a kind of cabinet of freedmen, on whom he bestowed honours, to superintend various branches of the administration. An impressive series of documents, such as a speech for the admission of Gauls to the Senate recorded on a partly defective inscription at Lugdunum (Lyon), the edict for the Anauni (an Alpine population who had usurped the rights of Roman citizenship and whom Claudius confirmed in these rights), and the aforementioned letter to the city of Alexandria (AD 41), survive as evidence of his personal style of government: pedantic, uninhibited, alternately humane and wrathful, and ultimately despotic. The inscription from Lugdunum is an interesting comparison with the version of the historian Tacitus in his Annals, which gives an account of the same speech. The speech as recorded in the inscription, in spite of irrelevance, inconsequence, and fondness for digression (much of which is absent in the version of Tacitus), shows that Claudius knew what he wanted and that he appreciated the latent forces of Roman tradition.

His marriage with Messalina (Messalina Valeria) ended in 48, when she apparently conspired against him and, according to Tacitus, conducted a public marriage ceremony with her lover, Gaius Silius. Messalina and Silius were killed, and Claudius married his niece Agrippina (Agrippina, Julia), an act contrary to Roman law, which he therefore changed. To satisfy Agrippina's lust for power, Claudius had to adopt her son Lucius Domitius Ahenobarbus (later the emperor Nero), to the disadvantage of his own son Britannicus. In addition, the new commander of the guards, Afranius Burrus (Burrus, Sextus Afranius), was protected by Agrippina. Roman tradition is unanimous in stating that Claudius was poisoned by Agrippina on Oct. 13, AD 54, though the details differ. A version of poisoning by mushrooms prevailed. Lucius Annaeus Seneca (Seneca, Lucius Annaeus), the politician and satirist, who had been exiled by Claudius at his accession but had been recalled at Agrippina's urging to educate Nero, derided the dead emperor and his apotheosis (duly decreed by the Senate) in the satire Apocolocyntosis divi Claudii (“The Pumpkinification of the Divine Claudius”; the title and its exact meaning are both subject to dispute).

The picture of Claudius that appears in this work has much in common with that of later Roman historians who give details of the unpopular side of Claudius' administration. The Apocolocyntosis ridicules his physical appearance and his speaking ability and casts aspersions on his abilities as a judge, depicting him as arbitrary—of giving legal judgments without a fair hearing and of summarily ordering the executions of relatives, senators, and knights.

Tacitus, Suetonius, and the later historian Dio Cassius attribute Claudius' mistakes to infirmity of character and the influence of his wives and freedmen. They echo the hostility of the upper classes against an emperor who, in spite of his words, had been unfavourable to them. That this tradition is one-sided is shown by the surviving documents of the reign and the energy with which Claudius carried out the affairs of government.