He was given singing lessons while in elementary school. His university studies were intended to prepare him for a career as a music teacher, and he worked as a schoolteacher for a time. But his developing vocal talents made it feasible to attempt a serious singing career. He embarked on additional voice training, teaching, and interpretation studies with Else Bischof-Bornes, Jakob Stämpfli, and Peter Massmann.
Mertens has established a major concert career and has made over 100 recordings. He has worked with leading interpreters in the early music/authentic performance school -- for example, Ton Koopman, with whom he participated in a complete set of the Bach cantatas, as well as René Jacobs, Philippe Herreweghe, Sigiswald Kuijken, Frans Brüggen, Gustav Leonhardt, and Nikolaus Harnoncourt. With conductors such as Gerard Schwarz, Peter Schreier, Edo de Waart, Herbert Blomstedt, Iván Fischer, and Hans Vonk he has appeared in major orchestral concerts, including those with the Saint Louis, Chicago, and Jerusalem Symphony Orchestras, the Dresden Philharmonic, the Leipzig Gewandhaus Orchestra, and the Royal Concertgebouw Orchestra of Amsterdam.
Since much of the activity in the early music field is associated with summer music festivals, he frequently appears at such venues as the Lucerne Festival, the BBC Proms, the Mostly Mozart Festival of New York, the Salzburg Festival, the Prague Spring, Bach-Tage Berlin, the Flanders Festival, the Oxford Handel Festival, and many others.
In common with many other early music specialists, Mertens also has an interest in major repertory of the 20th century, performing Hindemith's A Requiem for Those We Love and Stravinsky's Oedipus Rex, among other works. He has also received praise for his work in Mendelssohn's Elijah, the same composer's Paul, and other major Romantic concert works. He also is highly regarded as a lieder singer. Mertens has frequently appeared on European television and radio broadcast concerts.