Felgen started his career as a teacher. During the Second World War, Felgen was a translator for the German occupiers, and then a reporter with a French-language newspaper. He studied theater and opera in Brussels and Liège; in 1946, he joined Radio Luxembourg as a chorus singer and a French-language reporter. In 1949, the mastering of his baritone completed his theatre and opera studies. In 1951, he had his first international hit record, "Bonjour les amies" ("Hello Friends"). The song went on to become the theme song for his national broadcaster. In 1953, he recorded his first German-language record, "Onkel Toms altes Boot" ("Uncle Tom's Old Boat"), in Berlin. He represented his home country in the Eurovision Song Contest 1960 with "So laang we's du do bast", becoming the first Luxembourger and the first male contestant to represent Luxembourg and the first participant to sing in Luxembourgish. He finished last with only one point. Two years later he entered the contest again, this time doing much better by finishing in 3rd place with the song "Petit bonhomme". He did at times do the commentary in German, too.