Gabriel García Márquez's life story is just as magical as anything in his fiction. He was raised in a tiny, largely illiterate town in an isolated region of a developing country, and his origins could hardly have been more obscure. His father was a philandering telegraphist, his mother bore 11 children, and he was left in the care of his eccentric grandparents. Though always prodigiously talented, he was so poor as a young man that he resorted to living in the attic of a whorehouse (apparently a purely economic decision).

And yet, by the age of 40, García Márquez had written a book, One Hundred Years of Solitude, that caught the imagination first of Latin America, and then the world. As Gerald Martin argues in this official biography, it became the "first truly global novel". Nor was it a one-off: a string of critically acclaimed and internationally bestselling books followed. García Márquez became the best-known practitioner of "magical realism", the style with which successive generations of authors have recalibrated the relationship between developing countries and their former colonisers. Martin argues, indeed, that he is the only indisputably great author of the late 20th century. He has won many accolades, including the Nobel Prize, and men of influence - Bill Clinton, Fidel Castro - have sought his friendship and confidence.

The most enjoyable sections here, perhaps inevitably, are those that focus on the early life of "Gabo" in the small town of Aracataca, on Colombia's Caribbean coast. Martin's research demonstrates just how much of the imaginary world that García Márquez would draw on was formed before he turned seven, in his grand parents' bare, gloomy house. His grandfather Colonel Nicolas Márquez was a local functionary and hero of one of Colombia's civil wars. He was a rational and educated man who doted on his young grandson. He provided a model for several famous characters; like Dr Juvenal Urbino in Love in the Time of Cholera, he died after falling while he retrieved the family parrot from a mango tree. Grandmother Tranquilina was a more terrifying prospect, a superstitious woman who ran her household according to signals she received through thunderstorms, ghostly black butterflies and other supernatural forces.

As a child, García Márquez absorbed his grandparents' contrasting world-views. His genius was to bring them together in his fiction, treating each with equal weight and respect; in magical realist literature, as Martin writes, "the world is as the characters believe it to be". It was this innovation - admittedly not Márquez's alone, as he drew on the work of Juan Rulfo, Alejo Carpentier and other Latin American authors - that proved so influential, as it implicitly challenged the colonial idea that there could only be one valid belief system. It is still an enormously popular way of writing about the developing world: the slow, rural communities that nourished García Márquez may have gone out of fashion with younger American-Latino authors such as Daniel Alarcón and Junot Díaz (their work, as the latter put it, is less "Macondo than McOndo"), but they continue to use the same technique.

Perhaps more than being a champion of the underdog, however, García Márquez has been an obsessive observer and documentor of the powerful. Following the surprise success of One Hundred Years of Solitude in 1967, he quickly set about converting his literary prestige into political influence, and a great deal of this long book is devoted to his deft navigation through the perilous waters of Latin American politics. He was not a young radical (although while studying in Bogotá he witnessed the "Bogotazo" riot in which, coincidentally, Castro, visiting Colombia as part of a student delegation, got his first taste of revolution). Only after travelling as a journalist in much of Latin America and both capitalist and communist Europe did he take a position. Once he had done so, however, he stuck to it - unlike many of his peers, notably his old rival Mario Vargas Llosa, who eventually stood as a centre-right presidential candidate in Peru.

This stubborn conviction has led him to take some apparently contradictory stands over the years - he founded an independent journalism foundation in Colombia but simultaneously supported Castro and his state-controlled press. Yet this book shows how skilfully García Márquez has operated as a negotiator and bridge-builder, sometimes in almost impossibly hostile circumstances. Other Latin American authors withdrew their support from Castro after the Cuban authorities stripped the dissident poet José Padilla of a literary award, but García Márquez held his tongue and patiently negotiated its reinstatement. He has at various times been in dispute with both Cuba and the United States, but still remained able to influence opinion on either side. Similarly in Colombia, he is one of the few public figures to command respect across the political spectrum.

It took Gerald Martin 17 years to write this book: there is apparently a 2,000-page unpublished version. It was originally an unofficial biography, but as time passed a friendship developed between the two and García Márquez eventually accepted Martin as "official". It occasionally feels as if, in the process, a degree of critical distance has been lost - for example, the marriage between García Márquez and his wife, Mercedes, is only ever described in the most glowing tones, to spare their feelings, one imagines. Nevertheless, it is both a fine tribute to a remarkable artist and a fascinating cultural history of the region he helped to find its voice.

Alice O'Keeffe is arts editor of the New Statesman