Louis ran the Paris branch, moving to the Rue de la Paix in 1899. He was responsible for some of the company's most celebrated designs, like the mystery clocks (a type of clock with a transparent dial and so named because its mechanism is hidden), fashionable wristwatches and exotic orientalist Art Deco designs, including the colorful "Tutti Frutti" jewels.
In 1904, the Brazilian pioneer aviator, Alberto Santos-Dumont complained to his friend Louis Cartier of the unreliability and impracticality of using pocket watches while flying. Cartier designed a flat wristwatch with a distinctive square bezel. This watch was favored not only by Santos-Dumont himself but also by many other customers. The "Santos" watch was Cartier's first men's wristwatch. In 1907, Cartier signed a contract with Edmond Jaeger, who agreed to exclusively supply the movements for Cartier watches. Among the Cartier team was Charles Jacqueau, who joined Louis Cartier in 1909 for the rest of his life, and Jeanne Toussaint, who was Director of Fine Jewellery from 1933.
On the other hand, Pierre Cartier established the New York City branch in 1909, moving in 1917 to 653 Fifth Avenue, the Neo-Renaissance mansion of Morton Freeman Plant (son of railroad tycoon Henry B. Plant) and designed by architect C.P.H. Gilbert. Cartier bought it from the Plants in exchange for $100 in cash and a double-stranded natural pearl necklace valued at the time at $1 million. By this time, Cartier had branches in London, New York and St. Petersburg and was quickly becoming one of the most successful watch companies in the world.
Designed by Louis Cartier, the Tank watch model was introduced in 1919 with a design inspired by the newly introduced tanks on the Western Front in World War I. In the early 1920s, Cartier formed a joint-stock company with Edward Jaeger (of Jaeger-LeCoultre) to produce movements solely for Cartier. Cartier continued to use movements from other makers: Vacheron Constantin, Audemars Piguet, Movado and LeCoultre. It was also during this period that Cartier began adding its own reference numbers to the watches it sold, usually by stamping a four-digit code on the underside of a lug. Jacques took charge of the London operation and eventually moved to the current address at New Bond Street.