Hydraulic brakes were on some cars by the 1920s. I know the American air-cooled Franklins had them in 1929 and Plymouth had them in the 1930s. Disc brakes were first used on aircraft. A friend who works on small planes, recently showed me a disc brake from a 1949 Piper Cub. Chrysler even offered a very unusual and complicated disc brake on early '50s Imperials. It was the British who pioneered automotive disc brakes as we know them, in the mid '50s, I believe on Jaguars. Those were more effective than drums but suffered from short pad life. The British Rootes Group, makers of Hillman and Sunbeam, experimented with discs on the prototype for their 1960 Sumbeam Alpine sportscar and came up with the splash shield to keep water and road grime from prematurely wearing the disc pads.
The W111 220SE coupe was the first production Mercedes to be fitted with British Girling disc brakes, in late 1960.

Happy Motoring, Mark
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DrDKW