We know nothing about her life prior to her marriage. She married Claudius in 38 CE, three years before he became Emperor. The couple had two children, the future Empress Claudia Octavia and Brittanicus,* the heir presumptive, born less than a month into Claudius’ reign.
Messalina quickly made herself a major player in imperial politics. Many courted her favour and tried to make use of her influence. Both Cassius Dio and Suetonius accuse her of colluding with at least one of the Emperor’s freedmen to have him put to death several of their political enemies, or in some cases, people who simply managed to annoy them.
Rumors of her voracious sexual appetite abound in the histories, though it is worth noting that most of them come from after her death. Several of the men she supposedly conspired to have executed were those who had supposedly refused her. Other men came willingly into her bed or had to be tricked.
Such stories, however, served a political purpose. For the supporters of Nero, who became the Emperor’s stepson when Claudius married Agrippina the Younger after Messalina’s death, they called into question not only whether Brittanicus was actually a legitimate heir to the Empire, but also whether or not he was a stable enough person to rule, given who his mother was. For the later historians from whom we hear them (political enemies of the people they wrote about), they serve to further highlight the depravity of the later Julio-Claudians.
In 48 CE, Messalina publically married Gaius Silius,** though whether this was because she was slave to her passions and madly in love with him or because she felt Silius would be a stronger political support to her son than the aging Claudius and plotted against her husband remains unknown. She was executed shortly after Claudius discovered what had happened.
*He was originally named Claudius Germanicus, but was given the name Brittanicus after his father’s conquest of Britain. It is by this name that he is best known.
**I don’t know about you, but I find his name… silly.
Tacitus, Annals, 11.2, 12; 11.26-38 - Lacus Curtius
Suetonius, Life of Claudius (sections 17, 26, 27, 29, 36, 37, 29) - Lacus Curtius
Cassius Dio, Roman History, 60.8, 60.12-18, 60.22, 60.27-28, 60.29-34 - Lacus Curtius
Valeria Messalina - De Imperatoribus Romanis
Messalina - Wikipedia