The Athletics either sold or traded his rights to the Cubs in June, and he made two more minor league stops in Ponca City and Lafayette.

He induced the first batter the faced, Roy McMillan, to ground out to third base, and struck out Eddie Kasko for the second out, his first major league strikeout.

The next batter was opposing pitcher Bob Purkey, who doubled for the first hit Donnelly surrendered in the big leagues, but he escaped the inning without giving up a run.

In Donnelly's final big league appearance (September 20), he threw a scoreless eighth inning as the Cubs fell to the St. Louis Cardinals.

Donnelly spent the next two seasons with the Cubs' AAA affiliate, the Houston Buffs, going a combined 15–10 in 101 games (100 relief appearances, 1 start) with ERAs of 3.00 and 3.36.