Marks left home as a youth, escorting horses to Sheffield in England, and chose not to go back to the persecution that shadowed Jewish life in the Russian Empire. In Sheffield he found work, in-laws, and eventually a new direction. When word reached him of the diamonds turning up at Kimberley, he sailed for the Cape in 1869, soon joined by his cousin Isaac Lewis from the same hometown. Together they formed Lewis & Marks, a partnership that would endure for decades. Marks began humbly, peddling goods through the rural Cape, then supplying the mines and diggers at Kimberley before moving into diamond trading itself. The fortune that followed was built less on luck at the diggings than on the patient business of selling to everyone who came to dig.

In 1881 Marks settled in Pretoria, capital of the Boer republic known as the Zuid-Afrikaansche Republiek, and there he formed the alliance that defined his career. He won the trust of President Paul Kruger, and the friendship between the immigrant Jewish financier and the stern Afrikaner patriarch grew genuinely close. They shared humble beginnings and a ready wit. Marks urged Kruger to build a railway from Pretoria to the sea at Lourenço Marques, a line Kruger came to see as vital to the republic's independence from British control. As gold transformed the highveld, Marks spread into coalfields, distilleries, a glass factory, brickworks, and steelmaking. He pioneered steam tractors on his Vaal River estates, and the mining company he helped found in 1892 gave the town of Vereeniging its name.

The closeness of the two men produced one of the strangest episodes in South African numismatic history. In 1898 Marks was granted the extraordinary privilege of using the state mint for a single day. He used it to strike 215 gold tickeys, the threepenny coin normally made of silver, as keepsakes for his family and friends, President Kruger among them. The gold almost certainly came from the Sheba mine near Barberton, the one goldfield in which Marks held a major stake. Today these tiny coins are prized rarities, but at the time the gesture said something larger: in the ZAR, the boundary between Marks's private wealth and the machinery of the state could blur until it nearly vanished. His generosity ran in other directions too. He paid for the bricks, chandeliers, and electric lighting of Pretoria's Old Synagogue, and he commissioned the towering bronze statue of Kruger that still stands on Church Square, sculpted by Anton van Wouw at a cost of ten thousand pounds.

East of Pretoria, on a farm named Zwartkoppies for the iron-dark rock that drew lightning to its hills, Marks built a home to match his standing. He is said to have sketched the first plans on a piece of wood and handed them to a local builder, an origin story that explains the house's most famous quirk: it faces west, a sensible orientation in the northern hemisphere but a chilly one south of the equator. The finished mansion ran to some forty rooms. An Italian painter decorated its walls on stretched silk so the rooms seemed sheathed in fabric. His wife Bertha ran the great household, raising their children, keeping a garden, and hosting a near-constant procession of luncheons, croquet parties, and five-course Sunday dinners for thirty or more, served by a London-hired staff. After the family at last departed and the house stood empty, it was rescued, restored, and opened in 1986 as the Sammy Marks Museum, now part of the Ditsong Museums of South Africa, where a brass band still plays in the gardens on Sundays.

Marks's reach extended well beyond Kruger's circle. He kept the confidence of the Boer generals Botha, De Wet, and de la Rey, and earned the respect of the British commanders Roberts, Kitchener, and Milner, an unusual feat for one man to manage on both sides of a bitter war. He played a real part in the negotiations that ended Anglo-Boer hostilities at Vereeniging in 1902. When the Union of South Africa was formed, he was named a senator in its first parliament, a seat he held until his death in 1920. Biographers later called him "the uncrowned king of the Transvaal," and the title fits a man who built much of a region's industry, befriended its presidents and its conquerors alike, and left behind a synagogue, a statue, and a silk-walled house that still draws visitors a century on.