After studies in Utrecht with the composer and conductor Richard Hol, the composer Anton Averkamp (1861–1934) and the violinist Henri Wilhelm Petri (1856–1914), Mengelberg went on to study piano and composition at the Cologne conservatory (now the Hochschule für Musik Köln), where his principal teachers were Franz Wüllner, Isidor Seiss and Adolf Jensen.

Mahler regularly visited The Netherlands to introduce his work to Dutch audiences, including also his First, Fifth, and Seventh Symphonies, as well as Das klagende Lied and Kindertotenlieder.

[2] Mengelberg also founded, in 1899, the annual Concertgebouw tradition of performing the St. Matthew Passion of Johann Sebastian Bach on Palm Sunday.

His biographer Frits Zwart wrote, for Radio Nederland, of an "interview Mengelberg had given with the Völkischer Beobachter, the German Nazi newspaper...the gist of it was that, on hearing of the Dutch surrender to the German invaders on 10 May 1940, he had brought a toast with a glass of champagne [and] had also spoken about the close bond existing between the Netherlands and Germany."

Zwart also notes that Mengelberg conducted in Germany and in German-occupied countries throughout the war, and was photographed in the company of Nazis such as Arthur Seyss-Inquart.

[14] The documentary also notes that Mengelberg wrote letters to the occupying authorities to attempt to prevent the deportation of up to 48 musicians and people of Jewish background to the Westerbork transit camp.

In 1943, the authorities ceased consideration of Mengelberg's requests, having noticed the high frequency of his communications of behalf of Dutch Jewish citizens.

[15] 34 of the people for whom Mengelberg advocated avoided deportation to Westerbork and survived the occupation and the war, including 13 musicians of the Concertgebouw Orchestra and Sara Bosmans-Benedicts, mother of Henriëtte Bosmans.

The Dutch veterinarian Aat Tromp, whose father Sam Tromp was one of the Jewish musicians of the Concertgebouw Orchestra on whose behalf Mengelberg communicated to the authorities and who had received protective treatment, has commented in the documentary: The New Grove Dictionary of Music and Musicians (1980) entry on Mengelberg describes him as a "martinet addicted to meticulous and voluble rehearsals";[17] it also notes that he did not hesitate to make what he called changements to a composer's scores when he felt it would aid clarity.

Mengelberg's recordings with the Concertgebouw Orchestra are marked by frequent use of an unusually prominent portamento, the sliding of the string players' left fingers from one note to another.

The scholar Robert Philip has shown that Mengelberg's recordings with other orchestras do not show this portamento, and that "the unusual approach to portamento... was a stylistic feature which he developed with [the Concertgebouw] over a long period of rehearsal, and that it was not a style which could be transferred to other orchestras when Mengelberg visited them" [18] Philip also notes that this portamento required the strings to use uniform fingering prescribed by Mengelberg, and that this was also unusual for the time, when much orchestral fingering was typically "free", with different players fingering a passage differently.

For example, the musicologist and music theorist Walter Frisch has argued that "in the Brahms performances recorded by Willem Mengelberg, tempo fluctuation too often tends to obscure the broader shape of a passage or movement".

[citation needed] Many of his recorded performances, including some live concerts in Amsterdam during World War II, have been reissued on LP and CD.

Sound films of Mengelberg conducting the Concertgebouw Orchestra, during live concerts in Amsterdam, have survived; among these are a 1931 performance of Weber's Oberon overture.