The Ballymena welterweight took on Algerian Zohir Kedache at the Riocentro Pavilion 6 and came away with a comfortable unanimous decision win.
In a dominant opening round, Donnelly was in and out of range like lightning, showing a sharpness that was surprising considering his two year absence from competitive 3×3 minute action.
30 year old African started well in the second, dragging the Antrim man into a fight, but Donnelly proved himself well capable of mixing it before reverting to his powerful counterpunching style and taking a 10-8 on one of the scorecards.
With the win near-secured, Donnelly coasted through the final round, doing enough to deter Kedache who was pouring forward in search of a Hail Mary.
With the result in no doubt, Donnelly took the fight on all three judges cards (30-27, 30-26, 29-28).
The Irishman will face powerful Mongolian Tüvshinbat Byambya, who defeated former Donnelly victim Alberto Palmetta of Argentina today, in the Last 16 on Thursday at approximately 16:45 Irish time.
Earlier today Andrey Zamkovoy, the Russian who helped send Donnelly to these Games, was sensationally eliminated by Kenyan Rayton Okwiri. Okwiri will now face Moroccan World champion Mohammed Rabii in an African derby the Last 16 – the the winner facing off against the winner of Donnelly v Byambya for a bronze medal.