The first Cadillac automobiles were the 1903 Model built in the last quarter, 1902. These were 2-seater "horseless carriages" powered by a reliable and sturdy 10 hp (7 kW) single-cylinder engine developed by Henry Martyn Leland and built by Leland and Faulconer Manufacturing Company of Detroit, of which Henry Leland was founder, vice-president and general manager.

Reformed as the Cadillac Automobile Company in August 1902, it began manufacturing the runabouts and named them "Cadillac" after the city's founder Antoine Laumet, the self-styled Antoine Laumet de La Mothe, sieur de Cadillac.

Cadillac's single-cylinder lineup was consolidated into two models for 1906, the short wheelbase Model K and long wheelbase Model M. Priced at $750 for the Model K or $950 for the Model M, 3,650 units were sold that year. The K and M were essentially similar to the Models E and F from 1905 but with updated bodies. Production and sale of the Models K and M continued in 1907 in both the Tulip and straight lined body styles. The Model M was offered with a delivery body in 1906 and again for the 1907 model year. The Model M continued only as a commercial delivery vehicle for 1908, priced at $950.