A left-hander born in Havana, Donoso stood 5 feet 11 inches (1.80 m) tall and weighed 160 pounds (73 kg).

Seven years later, at age 32, his "organized baseball" tenure began when he won 19 games and lost only eight, with an excellent 2.37 earned run average, for the 1954 Hollywood Stars of the Pacific Coast League, an Open Classification circuit at the highest level of minor league baseball.

In his debut June 18, he allowed only two earned runs to the heavy-hitting Cincinnati Redlegs in a starting assignment at Crosley Field.

[5] Those three games account for 75 percent of Donoso's MLB victories: his only other triumph came July 4 against the New York Giants, with a three-inning, one-hit, shutout relief appearance at Forbes Field, as Pittsburgh won, 4–3.

In his 28 MLB games, Donoso compiled a 4–6 won–lost record, with one save, and a 5.21 earned run average in 962⁄3 innings pitched.