Howard Marks, a Welsh drug trafficker who at his peak in the 1970s controlled a substantial fraction of the world’s hashish and marijuana trade, and who became a best-selling author after his release from a United States federal prison, died on Sunday. He was 70.

His death, from colorectal cancer, which he disclosed last year, was confirmed by Robin Harvie, publisher for nonfiction at Pan Macmillan, the publishing house that released Mr. Marks’s final book, “Mr. Smiley: My Last Pill and Testament,” in September.

Mr. Marks’s drug-smuggling career started at Oxford University, where he studied physics in the 1960s and peddled marijuana on the side.

According to his 1996 autobiography, “Mr. Nice” (one of the many aliases he used), his induction into the drug trade followed a chance encounter with a Pakistani supplier.

He eventually teamed up with James McCann, an Irish Republican Army operative who arranged for large shipments of hashish through Ireland, which Mr. Marks and his accomplices then laundered through a staggering array of front companies.

In 1973, their activities widened to include the United States and Canada.

Mr. Marks was arrested on drug charges in Nevada in 1976, but he failed to appear in court and fled. He was eventually arrested in the Scottish Highlands, and in 1980 he faced narcotics charges in London.

To the government’s embarrassment, he was acquitted at his trial after arguing that he had been an agent of MI6, the British equivalent of the C.I.A. (In fact, his relationship with the agency had ended years earlier.)

Those escapades inspired a 1984 book, “High Time: Life and Times of Howard Marks,” by the investigative journalist David Leigh, which portrayed Mr. Marks as an aficionado of expensive suits who had lived as a playboy in London and New York.

It was the United States that eventually brought him to justice. In July 1988, Mr. Marks and his wife, the former Judith Lane, were arrested on the Spanish island of Majorca and were charged with 20 others.

They were accused of involvement in a drug-smuggling ring that encompassed — along with Britain, Canada and the United States — Australia, Hong Kong, the Netherlands, Pakistan, the Philippines, Portugal, Singapore, Spain, Thailand and West Germany.

The authorities seized more than $9 million in cash from the group, in addition to properties including a 103-foot-long boat in Vancouver, British Columbia.

“Mr. Marks was the Marco Polo of the drug traffic,” Thomas V. Cash, the special agent in charge of the Miami division of the Drug Enforcement Administration, said at the time. “He perfected smuggling methods and intricate laundering operations involving many countries around the globe, and this is why it took efforts in so many countries to complete this case.”

According to the indictment, Mr. Marks’s network smuggled “thousands of tons” of marijuana and hashish into the United States and Canada from 1973 to 1988. Sentenced to 25 years in prison, Mr. Marks was incarcerated in a high-security federal penitentiary in Terre Haute, Ind., before he was released in 1995.

Returning to Britain, Mr. Marks capitalized on his notoriety, writing his autobiography, the first of several books he would publish, including a novel.

The autobiography was the basis for a 2011 film, also called “Mr. Nice,” starring the Welsh actor Rhys Ifans.

“Those of us who were old enough in the 1960s and early ’70s to recall the smug, superior attitude (tinged with paranoia) of the period’s hipoisie will recognize his type and wonder exactly what happened to all those Mr. Tambourine Men preaching drugs, sex and rock ’n’ roll,” The New York Times critic Stephen Holden wrote in his review of the movie.

Mr. Marks also ran for Parliament, unsuccessfully, in 1997 on a single-issue platform of cannabis legalization.

Mr. Marks was twice divorced; his second wife, Judy Marks, wrote her own memoir, “Mr. Nice and Mrs. Marks: Life With Howard,” in 2006. He is survived by four children, including his daughter Amber Marks, a lawyer and author who co-directs the Criminal Justice Center at Queen Mary University of London.