His father died when he was only 3 years old in a boating accident, leaving him and his two older siblings to primarily be raised by their mother. Upon his death, their mother moved her young family to St. Remey-de-Provence, France in order to minimize their exposure to the press, giving them a chance at a normal childhood.
He attended local schools there until 1998-1999, when his life changed once again upon his mother's remarriage. After the marriage, the new family moved to the Parisian suburb of Fontainebleau, where Pierre and his siblings began attending school. Pierre graduated from Lycée François Couperin, Fontainebleau in 2005, the year after his sister Charlotte. After passing his bacclaurate, he took a year off as a gap year. In the autumn of 2006, he began an economics course at Bocconi University, where it has been said that he failed the first year, repeated it, did a second year, then dropped out in 2009 or so. It was widely believed that he was studying business to eventually take over his father's businesses that are presently managed by his paternal grandmother and uncle. In June 2009, he became the majority shareholder of the Monaco-based construction company his father founded in 1984. His uncle, Marco Casiraghi, remains in the position of President over this company, which is Engeco S.A. The acquisition of the majority shares to Casiraghi's name was handled by the Milan-based legal firm Bird & Bird.
He was linked to a number of young women during his university years in Milan. He was photographed out and about with the fashion models. But Casiraghi's longest and most serious romantic attachment to date has been since May 2008 the aristocratic and outgoing Columbia University Politics student Beatrice Borromeo.
In February 2012, he was briefly hospitalized following a physical altercation at a Manhattan nightclub. According to media reports he approached the table of businessman Adam Hock, models Natasha Poly, Anja Rubik and Valentina Zalayeva. Words were exchanged and an altercation ensued. Hock was arrested on four counts of assault, and he is alleged to have struck Casiraghi as well as three male friends, Adam Hock contends Casiraghi was the aggressor.