KNOWN as the “Builder Prince” because he transformed his tiny Mediterranean principality from the backwater that Somerset Maugham called a “sunny place for shady people” into a contemporary city-state, Prince Rainier III ruled his 482-acre fiefdom in a soft but determined fashion during his half century in power.

Under his guidance, Europe’s second-smallest independent country (after the Vatican) became a corporate banking, convention and tourism centre. Located 12 miles from the Italian frontier and bounded by several districts of the French Maritime Alps, Monaco became an intriguing mix of quaint cobblestoned streets, luxurious casinos, lavish hotels, high-rise apartment residences and numerous subterranean wonders, all dominated by a salmon-coloured palace.

When he acceded to power at the age of 25, Rainier III became the 31st member of the House of Grimaldi to run Monaco. The dynasty had begun when his ancestor, François Grimaldi, known as François the Spiteful, arrived disguised as a monk to take over the seaside territory from the Genoese in 1297. The Grimaldis, except during brief periods such as 1793-1814, have ruled Monaco ever since.

Stocky, silver-haired and mustachioed, Rainier III had some 140 titles — among them were the Duke of Valentinois, Marquis of Baux, the Count of Carlades and the Lord of Matignon — but he was affectionately called “the Boss” by his son and heir, Prince Albert, and by other subjects in the principality.

Throughout his reign, Rainier demonstrated a political astuteness that enabled him to prevail against his own National Assembly, a French president and a Greek tycoon.

He will also be remembered for his flamboyant, fairytale marriage to the actress Grace Kelly in 1956. The royal prince and the Hollywood filmstar boosted the jet-set allure of Monaco on the international scene until Princess Grace died from injuries sustained in a car accident in 1982. The exciting image was further enhanced by two glamorous daughters, the Princesses Caroline and Stéphanie, with colourful personal lives that continue to fascinate glossy magazines.

Rainier led Monaco into a variety of international organisations, including the United Nations, which his country joined in 1993. He was the architect of contemporary Monaco in more than one sense. He physically increased the principality’s area by a fifth by land reclamation from the Mediterranean. His public works programme created an array of light industrial, business, cultural, athletic and residential projects. And more recent undertakings overseen by Rainier included a new cultural and convention centre and additional tunnels linking Monaco with France. It is a measure of the diversification and growth that gambling, which accounted for 95 per cent of the country’s income when Rainier assumed power, now brings in less than 3 per cent of its annual revenue.

Rainier Louis Henri Maxence Bertrand Grimaldi was born in the Palais Princier in Monaco, the son of Her Serene Highness Princess Charlotte Louise Juliette, Duchess of Valentinois, and His Serene Highness Prince Pierre Marie Xavier Antoine Melchior, the Count de Polignac, who had become a Grimaldi by sovereign decree.

Rainier, whose first name had been used for a hereditary Prince for more than 500 years of Grimaldi history, was the first Grimaldi heir since Honoré IV in 1758 to be a native Monégasque. His parents obtained a civil divorce in 1929, and Rainier’s grandfather and Monaco’s ruler, Prince Louis II, prohibited de Polignac from returning to Monaco and ultimately taking over the reins of the country.

Rainier studied in England at St Leonards-on-Sea, Summer Fields and Stowe before attending Le Rosey in Rolle, Switzerland. He left Le Rosey in 1939 and began his university studies at Montpellier University, where he took his BA in June 1943. He went on to study at the École Libre des Sciences Politiques in Paris. He was officially made direct heir to the throne in June 1944 after he celebrated his 21st birthday and his mother renounced her right to succession.

Rainier enlisted as a foreign volunteer in the Free French Army on September 28, 1944, and was posted to the headquarters of the Second Army Corps, commanded by General de Monsabert. As a second lieutenant he took part in the Alsace campaign and was cited for bravery under fire. He was awarded the Croix de Guerre and the Bronze Star. After the war he was promoted to lieutenant and posted to the economic section of the French military mission in Berlin. On January 16, 1947, he was awarded the rank of Chevalier in the Legion of Honour by Léon Blum, President of the provisional Government of the French Republic, before he was decommissioned and returned to Monaco. In April 1949 the French Government promoted him to the rank of captain and later awarded him the rank of colonel in December 1954.

On May 9, 1949, Rainier III succeeded his grandfather as sovereign prince, the formal coronation following on April 11, 1950. Almost immediately Rainier devoted himself to the development of industry, tourism, scientific research, sport and culture. Unlike other Grimaldis, who spent much of the year away from isolated Monaco, Rainier lived and reigned from the palace located on the Rock, the elevated mountain in Monaco which contains its original fortification with ramparts, gardens and the cathedral. Monaco’s National Day was celebrated on November 19, because that was Prince Rainier III’s saint’s day.

Being Monaco’s constitutional monarch was not a job to take lightly. The reigning prince is said to derive his power from God, and the country’s constitution stipulates that “executive power derives from the high authority of the reigning prince” who not only represents Monaco in all its dealings with foreign powers but also signs and ratifies all treaties. Legal power belongs to the sovereign who delegates the full exercise of it to the courts and tribunals.