Casiraghi was born on 5 September 1987 at Princess Grace Hospital Centre, in La Colle, Monaco. He is named after his maternal great-grandfather, Prince Pierre, Duke of Valentinois, his maternal grandfather, Prince Rainier III, and his father. His godparents are his uncle, Prince Albert II, and his aunt by marriage Laura Sabatini Casiraghi, the wife of his uncle Daniele. Casiraghi was three years old at the time of his father’s death in a boating accident. He spent his earliest years, after his father’s death, living at his mother’s farmhouse in the town of Saint-Rémy-de-Provence.

In June 2009, Casiraghi became the majority shareholder of the Monaco-based construction company Engeco, which his father founded in 1984. His uncle Marco remains in the position of President over Engeco. The acquisition of the majority shares to Casiraghi’s name was handled by the Milan-based legal firm Bird & Bird. Casiraghi is also the majority shareholder of Monacair.

In 2011, Casiraghi was inducted as an honorary member of the Jeune Chambre Economique de Monaco (JCEM).

In February 2012, Casiraghi was briefly hospitalized following a physical altercation at a New York nightclub. According to media reports, Casiraghi approached the table of businessman Adam Hock, models Natasha Poly, Anja Rubik and Valentina Zelyaeva, where words were exchanged and an altercation ensued. Hock, who was later charged on eight counts of misdemeanor assault, alleges that Casiraghi was the aggressor and that he struck Casiraghi and his companions in self-defence. Casiraghi was treated at New York Presbyterian Hospital for a laceration, bruising, and swelling of his face.

Casiraghi is a participant in the sport of inshore racing, which differs from offshore racing in that it is done along the coastlines. In January 2014, he was part of the Masarati team in the Cape2Rio competition. His team completed the journey from Cape Town to Rio in a record 10 days, 11 hours. 29 minutes, and 57 seconds. Casiraghi was a crew member of the Monaco Racing Fleet captained by Tommaso Chieffi which was first to cross the finish line in the 2013 edition of the Palermo-Monte Carlo Regatta. Casiraghi and the Monaco Racing Fleet also won the 10th edition of the Palmero-Monte Carlo race in August 2014., Casiraghi’s boat, the Esimit Europa 2, was the winner of the Giraglia Rolex Cup in June 2014.

Casiraghi is a fixture in the Monte-Carlo Historic Rally, a vintage automobile race that occurs every year in January. In May 2014, he competed in a professional auto race for the first time at the Volkswagen Scirocco R-Cup in Hockenheim. He was a celebrity guest starter in the race, driving car #26, and finished 22nd in the first race and 20th in the second race. He said he was encouraged to enter the race by family friend and racing legend Jacky Ickx.

Casiraghi has been in a relationship with the journalist Beatrice Borromeo since May 2008. They were married in a civil ceremony on 25 July 2015 in the gardens of the Prince’s Palace of Monaco. The religious ceremony took place on Isola Bella on 1 August 2015. Among the wedding guests were John Elkann, Lapo Elkann and Diane von Fürstenberg. Pierre and Beatrice’s first child, Stefano Ercole Carlo, was born on 28 February 2017. Their second child, Francesco Carlo Albert was born on 21 May 2018.

In August 2017, Casiraghi paired with Boris Herrmann to sail in the Fastnet Race. Starting at Cowes, moving up to Ireland and then back down to Devonshire, they finished the race in third place.