Ansseau started his studies as a baritone but moved into the tenor repertoire, though always retaining a strong lower register that served him well in Wagner and heroic roles later in his career. He was acclaimed for his rich timbre and had a very pleasing stage presence. At times, he lacked strong rhythmic emphasis, which sometimes lessened his dramatic impact, but certain operas, such as Massenet's Werther and Auber's La muette de Portici, brought out his best, creating incisive and powerful portrayals.
His father was the organist in the church, and Ansseau's interest in music was further developed by singing in the choir. He studied at the École de Musique in Dour, and then auditioned for the Brussels Conservatory, where he was provisionally accepted as a baritone. After two years, his teacher, Desire Demest, decided that Ansseau's growing strength in his upper ranges indicated that he was becoming a tenor, directing him towards that repertoire and to study with Ernest van Dyck, himself a retired tenor. Ansseau won first prize at the Conservatoire and made his debut in 1913 as Jean in Massenet's Hérodiade at Dijon. He did not sing staged opera during the World War I years, but after the war, became a regular lead at the Theatre Royal de la Monnaie. His Covent Garden debut was in 1919 as Des Grieux in Massenet's Manon, and his Monte Carlo debut followed two years later, again in Hérodiade. 1921 was also the year of his Opera-Comique debut in the title role of Massenet's Werther, and he made his Paris Opera debut in 1922 in Hérodiade. His United States debut was at the Chicago Opera in 1923, and in the 1925 - 1926 season, he went on tour with that company. He debuted in San Francisco in 1925 as Saint-Saëns' Samson. During the 1920s, he added heavier roles to his repertoire, including Don José in Bizet's Carmen, Cavaradossi in Tosca, and even Wagner's Tannhäuser and Lohengrin. His last staged performance was as Canio at La Monnaie in 1939, his last concert appearance in 1940. From 1942 to 1944, he taught at the Brussels Conservatory, and then retired from music completely.