The 23-year-old had been on the sidelines since losing his seat with Haas on the eve of the 2022 F1 campaign after Russia's invasion of Ukraine, but made his sportscar debut in the first of four four-hour races to be held over the next fortnight on Saturday.
Mazepin teamed up in 99 Racing's ORECA-Gibson 07 with 2016 Le Mans 24 Hours winner Neel Jani and Portuguese Goncalo Gomes, the latter a late replacement for Felix Porteiro when the two-time World Touring Car Championship race winner had his Bronze driver rating upgraded to Silver.
Mazepin held the lead with around 80 minutes to go until the crew lost ground under a full course yellow inserting Jani, who was brought into the lineup on the eve of the event after the previously announced Ben Barnicoat withdrew citing clashing commitments.
Former Porsche factory driver Jani came just short of beating Charlie Eastwood's DKR Engineering ORECA to second at the line.
The trio then finished sixth in race two and sit fifth in the standings heading into next weekend's races at the Yas Marina Circuit in Portimao, with the prospect of automatic entries for the Le Mans 24 Hours open to the champion in each division.
The Algarve Pro Racing machine shared by James Allen, Kyffin Simpson and John Falb currently leads the standings after taking victory in race one, over DKR drivers Eastwood, Ayhancan Guven and Salih Yoluc, who finished second in both races.
Race two spoils were claimed by the Inter Europol Competition ORECA hustled by American drivers Nolan Siegel, Christian Bogle and CR Crews, who recovered strongly from a spin, while Allen, Simpson and Falb took fourth.
The Polish-entered Inter Europol machine claimed pole for both races and ran strongly in the opener with impressive 18-year-old Siegel hassling Jani until a suspected wheel hub failure put him out.
LMP3 class honours were shared between the #29 MV2S Racing Ligier and #5 DKR Duqueine, while Walkenhorst Motorsport scooped both GT victories with its BMW M4 GT3.
Nicky Catsburg, Chandler Hull and Thomas Merrill headed home the Getspeed Mercedes-AMG GT3 of Raffaele Marciello, Fabian Schiller and Florian Scholze in both races.
