Context: It was said in 12 Against the Gods which was published in 1929.
There is a more subtle tragedy that waits for adventurers than ruin, penurious old age, rags, contempt. It is that he is doomed to cease to be an adventurer. The law of his morphology is that, setting out a butterfly, he is condemned when his development is ripe to become a caterpillar. The vocation of adventure is as tragic as that of Youth ; its course is parabolic, not straight; so that at a certain point it leads back to the cage again. The greatest adventurer that ever lived ended as a nervous, banal millionaire. joshvahvmphreys.com/
Google books has a more extensive quote, but google books makes it nearly impossible to reference the quote. Bolitho repeatedly refers to "the adventurer" as an abstract figure, not as a specific individual.
There appears to be an argument that this is L Ron Hubbard, but quite frankly I haven't the patience to read any argument that includes the name L Ron Hubbard. There are better things to do with my time. There are counterarguments at GerryArmstrong
I vote for Cecil Rhodes.
First, the author, William Bolitho Ryall was a South African, as was Rhodes.
Second, Rhodes was mightier than most of the others, insofar as he had a (former) country, "Rhodesia" named after him. (Alexander the Great "only" had cities like Alexandria named after him; Napoleon, "nothing".) Although technically just a wealthy private citizen, Rhodes basically ran his personal fortune as a "supranational" entity that dabbled in international affairs. (Think of George Soros and his private "war" with the Bank of England in the early 1990s.) Rhodes hired a mercenary force that basically started the Boer War.
On the more constructive side, he conceived the idea of a "Cape to Cairo" transcontinental railway under British control. He is the reason why the British empire in Africa extends north from South Africa to the future Bechuanaland (Botswana) and Rhodesia (Zimbabwe, Zambia). This became a reality after World War I when Britain captured intervening German territory in modern Kenya, Uganda, and Tanzania to connect to Egypt and the Sudan.
The winding down of the Boer War coincided with his personal "twilight," and his last public acts were "academic," establishing the "Rhodes scholarship" and donating land to the University of Cape Town. After the high drama of this earlier life, the substantial realization of his goals (and the slight chance before his death that they might fail), made him a "nervous, banal, millionaire."