In modern Britain, an exercise in thought association around the phrase ‘management consultant’ is likely to lead to a diatribe about wasted public spending and weak government. Management consultants are those masters of the bullet-point presentation who get rich by selling Whitehall departments the ‘strategies’ that enable ministers to chant the mantra of change over and over again, though rarely to useful effect.

But despite this image, a handful of consultancy brands continue to command a high level of respect in the corridors and boardrooms of power. Primus inter pares is McKinsey & Company. The company has enjoyed ‘pretty much uninterrupted access to 10 Downing Street for decades’, according to Duff McDonald, a position that is underpinned by a network of ‘alumni’ that includes, among many others, Foreign Secretary William Hague, former FSA chairman and CBI chief Adair Turner and former trade minister and HSBC chairman Stephen Green.

The enduring strength of the McKinsey brand is the enigma at the core of this well-paced corporate biography – a more interesting and provocative read than its subject and cover might suggest. Like the investment bank Goldman Sachs, McKin-sey maintains its mystique by refusing to talk about itself; its senior practitioners are largely unknown to the wider world. New York-based journalist McDonald has pierced the veil, avoiding the prime fault of American investigative writing – the tendency for fly-on-the-wall details to overwhelm narrative thread – while providing just enough gossip to light up his story. But even he struggles to explain why McKinsey remains so eminent, and so smugly self-confident, when trends and events have conspired for years to knock it off its pedestal.

The profession pioneered by James O McKinsey (who set up shop in Chicago in 1926) reached its zenith in the postwar decades, when it made its mark as the most valued adviser to the chief executives of great corporations – first in America, then in Britain and the rest of the world. Having opened a London office in 1959, McKinsey men wrote management blueprints for everyone from Tate & Lyle and ICI to the Bank of England and the BBC. If they sold pretty much the same blueprint to everyone – divisional structures with devolved decision-making replacing the centralised autocracies of old – they did so with authority and polish. For a company or institution to have been ‘McKinseyed’ gave it a certain seal of quality.

But that was in an era when chief executives strove for long-term success and peer-group esteem, rather than short-term shareholder gain. When capitalism began to mutate in the 1980s, the sober-suited McKinsey man, schooled not to be greedy for himself or the firm but to devote himself to his client’s best interests, was all too likely to be elbowed aside by faster-talking Wall Street and City dealmakers who epitomised greed and urged it on everyone else.

As for ‘The Firm’, it grew and grew, endangering the ethos of its guiding light, Marvin Bower, who led it from 1950 to 1967 and remained a powerful influence until his death, aged 99, in 2003. He wanted it to remain quasi-monastic in its integrity: small, frugal and self-effacing, offering fiercely honest and totally confidential advice. But under the later leadership of Rajat Gupta, who was managing director for the last decade of Bower’s life, it seemed to lose its way. Not only did it become big and bloated, but its people also became, like so many others in the upper levels of the modern corporate world, what Bower reportedly called ‘greedy fucks’. As if to prove his point, Gupta and another McKinsey director, Anil Kumar, were eventually convicted of insider trading, in collusion with a hedge-fund manager.

But how badly did that reputational crash damage McKinsey? Hardly at all, apparently. ‘For the most part, clients didn’t seem to care,’ writes McDonald, ‘or they even felt sympathetic.’ The firm and its fee revenues continue to grow. Clients don’t even seem bothered by the absence of an authentic list of McKinsey’s greatest hits. The firm seeks no credit (and accepts no blame) for what its clients do with the advice they receive. Even the rather vague assertion that McKinsey ‘reinvented the idea of American capitalism’ might be interpreted by critics to mean that they made capitalism leaner by giving bosses reasons to abolish millions of jobs.

And worse, McDonald suggests that McKinsey’s work must have underpinned, or been used to justify, key decisions taken in the boardrooms of many companies that have gone astray: at General Motors in the period when it failed to comprehend the nature of the competitive threat from Japan; at Enron, the scandal-torn energy trading giant; and in many of the banks that were at the heart of the financial collapse in 2008:

Whatever the specific advice to specific clients, McKinsey had once again failed to give them the best advice of all, which was to go in the direction of less, not more. That makes the value of the advice it did give incidental at best, and destructive at worst.

Using the opposing spirits of Rajat Gupta and Marvin Bower to create dramatic structure, McDonald skilfully sets up the mystery of a firm that has ‘no legendary consulting engagements … only legendary client relationships, the kind that keep money pouring in the door’. British readers will be left wanting to know precisely what value accrued to taxpayers from the £30 million paid to McKinsey by the Labour government between 2006 and 2010, and which NHS reforms have resulted from multiple McKinsey reports commissioned by the current coalition. And the unravelling of the mystery itself is left as an assignment for future management consultants. All we can conclude from Duff McDonald’s study is that the power of the strongest brands to reinforce the self-image of the loyal consumer or client can become strangely detached from the substance of what that brand offers.