Although his career was based in Munich, he appeared internationally as a guest singer at major opera houses and festivals and performed and recorded many of Bach's Passion oratorios and cantatas, primarily with the conductor Karl Richter.

When he returned to California he sang in local concerts and recitals and worked in a warehouse and as a chorus assistant to save enough money to train as an opera singer in Europe.

Engen entered Vienna's Wiener Musikakademie in 1951 where he studied under Elisabeth Radó, Tino Pattiera, and the bass Pavel Ludikar, who also became his friend and role model.

[3] Engen made his operatic debut in 1952 at the Graz Opera as Monterone in Rigoletto and sang various roles with the company during that season, including Zaccaria in Nabucco.

In addition to the title role in Bluebeard's Castle and Zaccaria in Nabucco, these included: the Mozart roles of Count Almaviva in Le nozze di Figaro, Sarastro in Die Zauberflöte, and the title role in Don Giovanni; Rocco and Don Fernando in Beethoven's Fidelio; Marke in Wagner's Tristan und Isolde and Hunding in his Die Walküre; Tommaso in D'Albert's Tiefland, La Roche in Richard Strauss's Capriccio and the Music Master in his Ariadne auf Naxos; the Sultan in Rossini's Il Turco in Italia; Enrico in Donizetti's Anna Bolena; Ramfis in Verdi's Aida; Mephisto in Gounod's Faust; and the Doctor in Alban Berg's Wozzeck.

During that season he also appeared there as Count Almaviva in Le nozze di Figaro, Don Fernando in Fidelio, Fritz Kothner in Die Meistersinger von Nürnberg , Varlaam in Boris Godunov, and Quince in A Midsummer Night's Dream.

[7][2][8] Engen took part in several world premieres, singing Emperor Rudolf in Hindemith's Die Harmonie der Welt (1957), Alexandre Dumas in Norman Dello Joio's Blood Moon (1961), Oceanus in Carl Orff's Prometheus (1968), Senatspräsident in Volker David Kirchner's Belshazar (1986), and the Czar in Penderecki's Ubu Rex (1991).

[7][a] Ronald Adler, former artistic director at the Bavarian State Opera and longtime friend of Engen, wrote that his personality was marked by unwavering benevolence, kindness, and serene wisdom with a focus that was always on the essential—love of music, life and people.