According to the 14th-century chronicle-writer Jean d'Outremeuse he was the son of Apre, lord of Loon, and his wife Herisplindis, both from noble families of Maastricht.

The child was baptized by his godfather, the local bishop Remaclus, and educated by Landoald, archpriest of the city and head of the noble abbey school in Wintershoven.

Lambert appears to have frequented the Merovingian court of King Childeric II, and was a protégé of his uncle, Theodard, who succeeded Remaclus as bishop of Maastricht.

He is described by early biographers as “a prudent young man of pleasing looks, courteous and well-behaved in his speech and manners, well-built, strong, a good fighter, clear-headed, affectionate, pure and humble, and fond of reading.” When Theodard was murdered soon after 669, the councillors of Childeric made Lambert bishop of Maastricht.

[1] After Childeric himself was murdered in 675, the faction of Ebroin, majordomo of Neustria and the power behind that throne, expelled him from his see, in favor of their candidate, Faramundus.

The bishop was murdered at Liège by the troops of Dodon, Pepin's domesticus (manager of state domains), father or brother of Alpaida.

Children build "Lambertus pyramids" of branches, decorated with lanterns and lamps around which they dance and sing traditional songs (known as Lambertussingen or Käskenspiel).