Rainier’s early education was conducted in England, at the public schools of Summerfields in St Leonards-on-Sea, Sussex, and later at Stowe School, in Buckinghamshire. Rainier then attended the Institut Le Rosey in Rolle and Gstaad, Switzerland from 1939, before continuing to the University of Montpellier in France, where he obtained a Bachelor of Arts degree in 1943, before studying Institut d’études politiques de Paris in Paris.

In 1944, upon his 21st birthday, Rainier’s mother renounced her right to the Monegasque throne and Rainier became Prince Louis’s direct heir. In World War II, Rainier joined the Free French Army in September 1944, and served under General de Monsabert as a second lieutenant. As soldier, he witnessed action during the German counter-offensive in Alsace. Rainier received the French Croix de Guerre with bronze star (representing a brigade level citation) and was given the rank of Legion of Honor in 1947. Following his decommission from the French Army, he was promoted by the French government as a Captain in April 1949 and a Colonel in December 1954.

Rainier became the Sovereign Prince of Monaco upon the death of Prince Louis, on 9 May 1949.

During his reign, the Societé Monégasque de Banques et de Métaux Précieux, a bank which held a significant amount of Monaco’s capital, was bankrupted by its investments in a media company in 1955, leading to the resignation of Monaco’s cabinet. In 1962, Rainier ratified the Principality’s new constitution, which significantly reduced the power of the sovereign. He had suspended the previous constitution in 1959, saying that it “has hindered the administrative and political life of the country. The changes ended autocratic rule, placing power with both the Prince and a National Council of eighteen elected members.

The Prince met Academy Award-winning actress, Grace Kelly in 1955, during a photocall at the Palace scheduled to support her trip to the Cannes Film Festival. After a year-long courtship described as containing “a good deal of rational appraisal on both sides”, Prince Rainier married Kelly in 1956. The union was met with mass attention from the public, described as the “wedding of the century” and the “world’s most anticipated wedding” by the media. The civil ceremony took place at the Palace on 18 April, with the religious wedding being held on 19 April at the Saint Nicholas Cathedral . Rainier wore a military dress of his own design, based on the uniforms of Napoleon Bonaparte. Presided over by Bishop Gilles Barthe, the marriage was broadcast by MGM Studios, and viewed by over 30 million people across the globe. The couple honeymooned in the Mediterranean on their yacht, Deo Juvante II.

Princess Grace gave birth to their first child, Princess Caroline, on 23 January 1957. Their second child and heir, Prince Albert, was born on 14 March 1958. Their youngest, Princess Stéphanie, was born 1 February 1965, with all children having been delivered at the Palace.

After ascending the throne, Rainier worked to recoup Monaco’s lustre, which had become tarnished through financial neglect and scandal (his mother, Princess Charlotte, took a noted jewel thief known as René the Cane as her lover). Upon ascension, the Prince held with a treasury that was practically empty. Monaco’s traditional gambling clientele, largely European aristocrats, found themselves with reduced funds after World War II. Other successful gambling centres had opened to compete with Monaco. To compensate for the loss of income, Rainier decided to promote Monaco as a tax haven, commercial center, real-estate development opportunity, and international tourist attraction. The early years of his reign saw the overweening involvement of the Greek shipping tycoon Aristotle Onassis, who took control of the Société des Bains de Mer and envisioned Monaco as solely a gambling resort. Prince Rainier regained control of SBM in 1964, effectively ensuring that his vision of Monaco would be implemented.

In 1979, the Prince made his acting debut alongside the Princess in a half-hour independent film, Rearranged, produced in Monaco. After its premiere in Monaco, Princess Grace showed it to ABC TV executives in New York in 1982, who expressed interest if extra scenes were shot. However, Grace died in a car crash caused by a stroke in 1982, making it impossible to expand the film for American release. After Grace’s death, Rainier refused to remarry. He established the Princess Grace Foundation-USA in 1982 in her honor, to support fledging American artists.

Rainier established a postal museum in 1950: the Museum of Stamps and Coins, in Monaco’s Fontvieille district by using the collections of the Monegasque princes Albert I and Louis II. The prestigious philatelic collectors club Club de Monte-Carlo de l’Élite de la Philatélie was established under his patronage in 1999; the club has its headquarters at the museum, with its membership restricted to institutions and one hundred prestigious collectors. Rainier organized exhibitions of rare and exceptional postage stamps and letters with the club’s members. Throughout his reign, Rainier surveyed all the process of creation of Monaco stamps. He preferred stamps printed in intaglio and the art of engravers Henri Cheffer and Czesław Słania.

Prince Rainier smoked 60 cigarettes a day. In the last years of his life his health progressively declined. He underwent surgery in late 1999 and 2000, and was hospitalized in November 2002 for a chest infection. He spent three weeks in hospital in January 2004 for what was described as general fatigue. In February 2004, he was hospitalized with a coronary lesion and a damaged blood vessel. In October he was again in hospital with a lung infection. In November of that year, Prince Albert appeared on CNN’s Larry King Live and told Larry King that his father was fine, though he was suffering from bronchitis.

On 7 March 2005, he was again hospitalized with a lung infection. Rainier was moved to the hospital’s intensive care unit on 22 March. One day later, on 23 March, it was announced he was on a ventilator, suffering from renal and heart failure. On 26 March, the palace reported that despite intensive ongoing efforts to improve the prince’s health, he was continuing to deteriorate; however, the following day, he was reported to be conscious, his heart and kidney conditions having stabilized. His prognosis remained “very reserved”.

On 31 March 2005, following consultation with the Crown Council of Monaco, the Palais Princier announced that Rainier’s son, Hereditary Prince Albert, would take over the duties of his father as regent since Rainier was no longer able to exercise his royal functions.

On 1 April 2005, the Palace announced that Rainier’s doctors believe his chances of recovery were “slim”. On 6 April, Prince Rainier died at the Cardiothoracic Centre of Monaco at 6:35 am local time at the age of 81. He was succeeded by his only son, who became Prince Albert II. He was buried on 15 April 2005 beside his wife, Princess Grace, at the Cathedral of Our Lady Immaculate, the traditional burial place of princes and princesses of Monaco, and the place where Prince Rainier and Princess Grace had been married in 1956. Because his death occurred shortly after that of Pope John Paul II, Rainier’s death was overshadowed in the media.