From the early 1950s to the 1980s, the many high dramas of American political life had Cohn present in some official capacity. At the astonishingly young age of 24 years and with little experience, Cohn prosecuted the Rosenbergs, a left-wing couple who were sent to the electric chair as Soviet spies.

Even at this early point in Cohn’s career, in the Rosenberg trial, ethical questions arose. The sensitivity of the case, for reasons to do with the divisions in American society, resulted in ex parte communications between Cohn as prosecutor and the trial judge during the case.

Cohn was later an aide to Senator Joe McCarthy. The patent unfairness of those proceedings, and the casual indifference to the personal consequences for individuals appearing before the tribunal were matters of no consequence to McCarthy, Cohn and others.

Cohn’s strategy and attitude in these high-profile cases demonstrated, in the context then and now, professional excess. Moreover, while engaged in these cases he was also in receipt of a salary as a partner in a firm of attorneys; “with another $7,500 from a mysterious source”.

The appalling unfairness around the McCarthy inquiry into communist influence are well-known; but reasserted and focused on Cohn’s activities, the history of the period must surely amount to an appalling condemnation of the legal profession for tolerating excesses.

In an FBI memo of July 1953 Cohn was assessed as “extremely intelligent and has an outstanding memory – but he is crude, brash and inexperienced … [he] is very dangerous”.

Stated differently; Cohn was completely out of control, and continued to be so.

It is almost certain on the extensive grounds set out in this excellent biography that Cohn knew what other individuals thought of him, but he did not care. Undiluted aggression, rudeness and an unflinching pursuit of an outcome irrespective of professional duties drove Cohn.

By his thirties, Cohn had begun working as an attorney for the Mafia. In his forties, he was an informal adviser to Richard Nixon. In his fifties, he partied with the glitterati at Studio 54 and visited the Reagan White House where he traded gossip with the first lady.

Somewhere in all this, Cohn carried on his legal practice. The other partners in the firm may have had to carry Cohn to a degree, and yet his clients were themselves often highly successful and his strategy of bullying often snatched victory from the apparent certainty of defeat.

Much the same might be said of his own indictments: he was tried three times for various fraudulent actions and, on each occasion, he was acquitted. Cohn ran risks in practice before these trials and ran risks at his own trials; and contrary to expectations he succeeded.

The luxurious New York premises of the firm, with Cohn’s executive home on the upper floor, seem most impressive but it is disturbing, or may be for some, to learn that from 1972 Cohn failed to renew the firm’s subscription to Westlaw, and the law books were all out of date.

Cohn’s aggression, his surly defence or defiance of authority, as required, were frequently flourished to conceal vast insecurities, particularly regarding his closeted sexuality. After the death of his mother (his father having died some years earlier) Cohn lost any inhibitions. Bird and Goldmark do not spare the reader many of the intimate details of the personal life of Cohn, in so far as that complicated part of the story can be discovered now, and a lot has been found. Cohn died in 1986 in distressing circumstances from AIDS-related complications.

Cohn was a fine example of an individual who, as the authors say, “poisoned the atmosphere” wherever he went. He bought and sold a number of companies as going concerns in a sort of business career running parallel to that of law. There came a point when all this and more was simply too much and Cohn was disbarred; not an easy task. He lived on but owed significant sums in unpaid income tax and after his death his business partners were left to sort out a big financial mess.

Notwithstanding the dislike that Bird and Goldmark patently have for Cohn, it is odd that he should be referred to throughout the book by his surname, or, just as often, as “Roy”. The title of the book also pulls a punch: Cohn was far, far beyond being just a “scoundrel”.

American Scoundrel: Roy Cohn’s Dark Journey from Joe McCarthy to Donald Trump by Kai Bird and Susan Goldmark. Published by Simon & Schuster, 528pp, £30.