In an 877 charter in the Capitulary of Quierzy, he possibly already appears as "Rainerus", alongside his probable father as one of the regents of the kingdom during Charles the Bald's absence on campaign in Italy.
[1] Reginar was lay abbot of important abbeys stretching from the Meuse (Dutch: Maas) to the Moselle through the Ardennes, Saint-Servais in Maastricht, Echternach, Stavelot-Malmedy, and Saint-Maximin in Trier.
Dudo of Saint-Quentin, in describing the great deeds of the early Normans, calls Reginar I (who, along with a prince of the Frisians named Radbod, was an opponent of Rollo, the founder of Normandy) a duke of both Hainaut and Hesbaye.
He and some other magnates who had been key to Zwentibold's election three years earlier then took the opportunity provided by the death of Odo of France to invite Charles the Simple to become king in Lotharingia.
Then, after the death of Gebhard in 910, in battle with the Magyars, Reginar led the magnates in opposing Conrad I of Germany and electing Charles the Simple as their king.