Publius Crassus served under Julius Caesar in Gaul from 58 to 56 BC. Too young to receive a formal commission from the senate, Publius distinguished himself as a commanding officer in campaigns among the Armorican nations (Brittany) and in Aquitania. He was highly regarded by Caesar and also by Cicero, who praised his speaking ability and good character. Upon his return to Rome, Publius married Cornelia Metella, the intellectually gifted daughter of the Metellus Scipio, and began his active political career as a monetalis and by providing a security force during his father's campaign for a second consulship.
Publius’s promising career was cut short when he died along with his father in an ill-conceived war against the Parthian Empire. Cornelia, with whom he probably had no children, then married the much older Pompeius Magnus ("Pompey the Great").
Scholarly opinion is divided as to whether Publius or his brother Marcus was the elder, but with Roman naming conventions, the eldest son almost always carries on his father's name, including the praenomen, or first name, while younger sons are named for a grandfather or uncle. The achievements of Publius, named after his grandfather (consul in 97 BC) and uncle, eclipse those of his brother to such an extent that some have questioned the traditional birth order. Both Ronald Syme and Elizabeth Rawson, however, have argued vigorously for a family dynamic that casts Marcus as the older but Publius as the more talented younger brother.
Publius grew up in a traditional household that was characterized by Plutarch in his Life of Crassus as stable and orderly. The biographer is often harshly critical of the elder Crassus's shortcomings, particularly moralizing his greed, but makes a point of contrasting the triumvir's family life. Despite his great wealth, Crassus is said to have avoided excess and luxury at home. Family meals were simple, and entertaining was generous but not ostentatious; Crassus chose his companions during leisure hours on the basis of personal friendship as well as political utility. Although the Crassi, as noble plebeians, would have displayed ancestral images in their atrium, they did not lay claim to a fictionalized genealogy that presumed divine or legendary ancestors, a practice not uncommon among the Roman nobility. The elder Crassus, even as the son of a consul and censor, had himself grown up in a modestly kept and multigenerational house; the passage of sumptuary laws had been among his father's political achievements.
In marrying the widow of his brother, who had been killed during the Sullan civil wars, Marcus Crassus observed an ancient Roman custom that had become old-fashioned in his own time. Publius, unlike many of his peers, had parents who remained married for nearly 35 years, until the elder Crassus's death; by contrast, Pompeius Magnus married five times and Julius Caesar at least three. Crassus remained married to Tertulla "despite attacks on her reputation." It was rumored that a family friend, Quintus Axius of Reate, was the biological father of one of her two sons. Plutarch reports a joke by Cicero that made reference to a strong resemblance between Axius and one of the boys.