Born in Piacenza, Merosi trained as a building surveyor, before he discovered his gift for the automotive engineering. He gained first experiences at Bianchi and the running department of Fiats. Soon he was hired (1910) as a new chief engineer to new company A.L.F.A which was established at Portello in Milan.
The predecessor company, which had manufactured licensed products of the French automaker Darracq, had failed. The first Alfa designed by Merosi was A.L.F.A 24 HP, which came on the market in 24 June 1910. From its 4.1 litres capacity were obtained a proud 42 hp and the cars reached 100km/h, at that time a very considerable maximum speed. In the first year already 50 pieces were sold. The cars proved to be durable and reliable vehicles with an elegant design. Higher HP models followed in the coming years. In 1914 Merosi designed the first Alfa Romeo DOHC engine, 4 cylinders, 4.5 litres and 16 valve head, this was used in 1914 Alfa Romeo Grand Prix car. World War I stopped development and it wasn't until 1922 Merosi came again with DOHC engine. The beginning of the 20's also saw the luxurious Alfa Romeo G1.
Merosi recognized also the meaning of racing both for the development and for the mark image. Thus already 1911 Alfa Romeo HP took part in the Targa Florio. The first pure racing cars were starting from 1920 the RL and RM as well as the P1. 1923 Targa Florio was won for the first time.