Canada declared war on Germany on August 4, 1914 while Ernest MacMillan enjoyed performances in Bayreuth, Germany. He then went to Nuremberg to study and compose. Believing he had registered as a visitor-tourist, he misunderstood the notices that German police had posted requiring enemy aliens to report to the police, but remained wary as his British colleagues were arrested starting in November.
In January 1915 he was arrested and fined 3,000 marks (depending on how calculated that is very roughly $25,000 CDN in today’s dollars) and sentenced to 2 months in prison.

Photo courtesy of Library and Archives Canada
Following prison he went to Ruhleben internment camp for British civilians near Berlin, where he became a musical leader of the 4,000-5,000 people (mostly men) held at a horse racing track including directing The Mikado and giving lectures on each of Beethoven’s nine symphonies.

Returning to Canada in January 1919 he became organist and choirmaster at Timothy Eaton Memorial Church in 1919 (staying there until 1926) and he immersed himself in musical life in Toronto.

He was named principal of the Toronto (later Royal) Conservatory of Music in 1926, holding that position until 1942.
He was named dean of the Faculty of Music in 1927, holding that position until 1952.
During his time at the Faculty, he oversaw:
- addition of music courses in 1934-35 year (MusBac and MusDoc degrees were awarded by exam only up to this point; academic courses were added following minor media hubbub of Percival Price, the Dominion Carilloneur at the Peace Tower in Ottawa, having his doctoral degree rejected by the Faculty),
- the approval by U of T of a 4 year BA in Music degree for Arts & Science students in 1937,
- a Music Education degree program established in 1946,
- and the administrative separation of the Royal Conservatory of Music and Faculty of Music in 1952 that ultimately led to his academic retirement.

He became conductor of the Toronto Symphony Orchestra in 1931 and held that position until 1956.

and its first with conductor Ernest MacMillan, photo courtesy of City of Toronto Archives
He became conductor of the Toronto Mendelssohn Choir in 1942, a position he held until 1956.
He was the first president of the Canadian Music Council in 1949, a position he held until 1966. This organization established the Canadian Music Centre in 1959.
He was the president of Composers, Authors and Publishers Association of Canada (CAPAC) from 1947 to 1969.
In 1935 at there mere age of 42 Ernest MacMillan was knighted by King George V “for services to music in Canada.”

Sir Ernest MacMillan died forty six years ago on May 6, 1973 (NY Times Obituary).
Additional resources:
Sir Ernest MacMillan (1893-1973), Library and Archives Canada
Sir Ernest MacMillan, The Canadian Encyclopedia
Sir Ernest MacMillan: The Importance of Being Canadian by Ezra Schabas (U of T Press, 1996)
MacMillan on Music, edited by Carl Morey (Dundurn, 1997)