Janssen came from a wealthy, music-loving family and received his first singing lessons in his early youth. He grew up in the family's castle on the Rhine, which was filled with magnificent art. His family wanted him to study law for the benefit of the family business. They disowned him upon discovering that he had used his law school tuition to study singing instead of law. He did, in fact, study law before deciding to commit to a professional singing career. The night he made his debut at the Berlin Stadstoper, a 12' Bosendorfer concert grand piano was delivered to the opera house with a card saying "welcome back to the family". He returned the piano to his family and went on to sing everywhere to great acclaim.
In 1922, Janssen was offered his first contract at the Berlin State Opera, starting with small roles but rising in status quickly. A year later, during the 1923-24 Berlin season, he appeared for the first time as Wolfram in Richard Wagner's Tannhäuser, a role that would become one of his trademarks.
Janssen remained a member of the State Opera's ensemble until 1937. During this time, he appeared as a guest at most of the important opera houses and festivals in Europe.
Beginning in 1925, Janssen spent the summer months singing at the Wagner-Festival at the Zoppoter Waldoper. From 1926 until World War II, he regularly sang at the Royal Opera House in Covent Garden, London. Guest appearances led him to the Vienna State Opera, Nationaltheater München, Opera Garnier in Paris, Semperoper in Dresden and the principal operatic theatres in Barcelona and Den Haag. From 1930 to 1937, he sang at the Bayreuth Festival.
He was known to say that he sang opera so he could sing Lieder. No one would attend a solo Lieder concert unless the artist had achieved fame in opera. He said he always considered himself a Lieder singer first and foremost. He made a number of recordings of Lieder, in addition to his sublime performances in opera, some of which have been preserved and are now available on CD.
Originally, Janssen had sung an extensive and manifold repertory. He appeared in, for example, Mozart roles (such as the Count in Le Nozze di Figaro) and as Lortzing's Zar Peter in Zar und Zimmermann. Major baritone roles composed by Giuseppe Verdi also figured in his early repertoire. They included Conte di Luna in Il Trovatore (a personal favourite of his) as well as Renato in Un ballo in maschera and Iago in Otello. He performed Bizet (Escamillo in Carmen), too, and much else.
Yet at the height of his career, especially at the Metropolitan Opera, Janssen was cast overwhelmingly in Wagnerian roles (a development which he regretted because it curtailed his versatility as a singer). Indeed, during his vocal prime, he was considered to be the most important extant singer of the more lyrical baritone parts in Wagner's music dramas. He was celebrated for his beautifully sung interpretations of Kurwenal (in Tristan und Isolde), Amfortas (Parsifal) and, above all, Wolfram (Tannhäuser). The heavier Wagnerian baritone roles, such as Wotan and Hans Sachs, were the natural preserve of Janssen's more heroic-voiced contemporaries Friedrich Schorr and Rudolf Bockelmann, but he was ill-advised enough to attempt them during the Second World War, owing to a shortage of dramatic singers at the Met.
Janssen made commercial gramophone records of some of his signature roles. There is also a recording derived from the 1930 Bayreuth Festival with him performing Wolfram's music, while he sang the role of Don Pisarro in a 1944 radio broadcast of Beethoven's Fidelio, with Arturo Toscanini conducting. These recordings have all been re-issued on CD. A live 1941 Met Tannhäuser reveals Janssen's perfection in the role of Wolfram, to which he brought unsurpassed humanity and vocal beauty. On top of his other attainments, Janssen was a fine Lieder singer, employing his soft, rich and velvety voice, with its Italianate timbre and smooth legato style, to outstanding effect.