The Belgian jockey was the retained rider for the Aga Khan until an incident occurred at Saint-Cloud in September involving Rossa Ryan.
Soumillon appeared to elbow Ryan out of the saddle during a race, causing him to fall, and subsequently served a 60-day suspension which led to him being released from his contract with the Aga Khan.
After missing stints in Hong Kong and Japan and international meetings such as Champions Day at Ascot and the Breeders' Cup, the 10-time French champion headed to South Africa to bounce back.
His first chance on Saturday's Kenilworth meeting came via Peter Muscutt's Isivunguvungu in the Franschhoek Motor Museum Winchester Sprint Cup, a five-furlong race that had attracted 15 contenders.
Placed in the middle of the group travelling on the outer rail, Soumillon mounted a late challenge aboard the fast-finishing gelding to sweep past all but one of his rivals ahead of the post.
Mr Cobbs, trained by Candice Bass-Robinson, was narrowly ahead at line, leaving Soumillon to miss out by a short head on his first ride since returning to the saddle.
The following Grade Two Cartier Sceptre Stakes did not wield a dissimilar result for the jockey, who again closed in on his rivals in the final strides of the race but had to settle for minor honours aboard Mike de Kock's third-placed Desert Miracle.
The feature contest of the meeting was the Grade One L'Ormarins King's Plate, a race in which Soumillon partnered Golden Ducat and came home seventh - after which he was unplaced again aboard Winter Air in the Glorious Goodwood Mr 74 Handicap.