When Herrmann announced in 2011 the first concert of the Rheingau Musik Festival's annual composer's portrait, featuring Hans Zender, he recalled that he was an altar boy at the church in Wiesbaden where Zender was the organist. In the 1960s, Herrmann attended the Pablo Casals Festival in Prades three times. Meeting important chamber musicians there, later singing in choral concerts in Eberbach Abbey in the early 1970s, inspired the idea of a music festival in the Rheingau.
Herrmann, who first learned bookselling, turned to tourism and worked in the Canary Islands for ten years. He met Christoph Eschenbach, Justus Frantz, Jürgen Ponto, Helmut Schmidt, Will Quadflieg and Leonard Bernstein. He returned to Germany in 1982 and worked for concert agencies. In 1985 he founded his own artists and concert agency.
He founded the Rheingau Musik Festival and, together with Tatiana von Metternich, Walter Fink and others, created in 1987 the Rheingau Musik Festival e.V. as an association which ran the festival until 1992, and has continued to support the festival. In 2010, it had 3300 members. Herrmann won sponsors who choose to fund their own concerts. It is normal for him to contact three to four sponsors a day. 180 companies have sponsored the festival. About half of the budget is funded by the sponsors and the remainder by ticket sales. Herrmann was successful in keeping the festival independent of public funding, but the minister-president of Hesse is its patron. Since 1992 the festival has been run by a GmbH (company with limited liability), with Herrmann as partner and managing director (Gesellschafter und Geschäftsführer). The festival has grown from 19 concerts in the two months of the first season to an average of 150 events each season, many of them in the Rheingau's vineyards and historical buildings, making the festival "one of the largest in Europe". Herrmann made the festival a member of the European Festivals Association and serves as the association's vice president.
The program is mostly dedicated to classical music, but also features cabaret, jazz, readings, musical cruises, children's concerts, wine tastings and culinary events with music. It is regularly opened by a symphony concert in Eberbach Abbey, broadcast live by Hessischer Rundfunk. Anniversaries of composers are celebrated, such as Giuseppe Verdi ("Viva Verdi") in 2001, and Franz Liszt and Gustav Mahler in 2011. Every year it has a theme, such as "Heimweh" in 2011. "Treffpunkt Jugend" (meeting point youth) presents soloists still in their teens in two marathon concerts of chamber music and concertos with orchestra. Work cycles, running over several years, have included the piano sonatas of Ludwig van Beethoven, played by Rudolf Buchbinder, and the symphonies of Anton Bruckner, with the WDR Symphony Orchestra Cologne and Eliahu Inbal. "Rendezvous" shows artists in dialogue. In the Rheingau, a traditionally Catholic region, the feast of the Assumption is celebrated on 15 August by a Marienvesper (Vespers for the Virgin Mary). In 2011 the Concerto Romano ensemble, conducted by Alessandro Quarta, performed a medley of works by composers from Rome. On this occasion, Hermann asked for an exceptional collection to benefit his guest of honor, bishop Ambroise of the diocese of Maradi in Niger. Organ concert on the historic instruments of the region are featured regularly. "Komponistenporträt" features annually a living composer in conversation, chamber music and symphonic music. Herrmann got Anne-Sophie Mutter to perform the German premiere of Wolfgang Rihm's violin concerto Lichtes Spiel, premiered in Avery Fisher Hall in 2010. He gave Andreas Scholl, born in the Rheingau, the opportunity to perform with friends and his sister Elisabeth in four of the area's historic churches. The festival usually concludes with a choral concert in Eberbach Abbey, including rarely performed works. In 2005 Frieder Bernius conducted Penderecki's Polish Requiem, Helmuth Rilling conducted Messa per Rossini in 2001 and Messiah of both Sven-David Sandström and Handel in 2009.
In the 25th anniversary season, he invited several of his long-term friend, called "Wegbegleiter" ("Companions along the way"), to appear again. In 2013, 160 events were offered at 45 locations, with a budget of 7,6 million Euro.
Since 2005 Herrmann has been vice president of the European Festivals Association. Since 2008 he has been director of the Pro Arte Konzertdirektion in Frankfurt.
In 1997 Herrmann was awarded the Hessian Order of Merit, and in 2002 received the Goethe Plaque, the highest award of the Hessian Ministry for Science and the Arts.