Carl Gustav Schmitt was born on 9 December 1837 at Frankfurt am Main, Germany, the fifth child of Aloys Schmitt, a celebrated composer, pianist and teacher, and his wife, Auguste Caroline Wohl. His uncle, Jacob Schmitt, and his brother, Georg Aloys, both made considerable reputations as musicians and composers, and his sister, Antonia, a pianist, married Julius Haast.
Schmitt received his early musical training from his father, under whose guidance he trained a youth orchestra. Later he studied violin and theory at the Frankfurter Musikschule, and at the age of 19 is said to have been appointed to a music director's post at Würzburg, and later held a similar position at Königsberg (Kaliningrad).
Schmitt came to Auckland, New Zealand, on the Breadalbane from Sydney, New South Wales, on 28 March 1859. He gave concerts and advertised as a teacher of violin, piano, singing, French and German before leaving for Tasmania, where he arrived early in 1860 and gave concerts as 'Herr W. Carl Schmitt, the great Bavarian Violinist'. Details of his Australian career are obscure, but in Melbourne on 23 April 1863, signing himself Gustav Wilhelm Carl Schmitt and stating his birthplace as Munich, he married Lucy Elizabeth Reeves. They were to have five children, two of whom died in infancy. In the late 1860s Schmitt was in Sydney, where he held various musical posts as organist and conductor, and established a choral society. He was also active as a composer, and his opera Cazille was performed there. Accepting a post in Tasmania in the 1870s, he became aide-de-camp and musical director to the governor, Frederick Weld.
In 1881 at the invitation of F. D. Fenton, Schmitt returned to Auckland to assume conductorship of the Auckland Choral Society. He also took up a position at the recently founded Ladies' College at Remuera. In 1884 the Auckland Amateur Opera Club was formed, and Schmitt was its founding conductor.
In 1888 the fledgeling Auckland University College established a chair in music, appointing Schmitt professor. The college and the Auckland Choral Society reached an agreement whereby each body contributed to Schmitt's salary. He was paid £100 per annum, while the college's four other professors were receiving salaries of £700 and were appointed for five-year terms against Schmitt's one year.
Schmitt's appointment provoked a public outcry. The New Zealand Herald dismissed him contemptuously as 'a society musician', and alleged his appointment to be a 'job' made on the grounds of personal friendship, citing the fact that Schmitt had no university qualification in music – an ill-informed criticism, as German universities did not offer degree courses in music at that time. The following year he was demoted from professor to lecturer. Nevertheless, he continued to style himself professor when engaged in outside activities such as conducting the Auckland Choral Society. Schmitt's tenure throughout his 12 years in office was never really secure. Until 1895 the music school was annually threatened with closure. Student numbers were small, and Schmitt appears to have been an ineffective administrator. Support for him came from Fenton, the choral society, and his Amateur Opera Club.
Schmitt was a prolific composer. He held a number of European honours, including an Italian knighthood, in recognition of musical works dedicated to European royalty. His compositions, few of which have survived, included the two-act opera Cazille, overtures, string quartets, and a setting of Psalm 95 for solo voice, chorus and orchestra. Sometime before 1874 he also composed the music for the national anthem of Tonga, still in use today.
Schmitt's programmes for the Auckland Choral Society drew heavily on the standard repertoire of Handel, Mendelssohn and early Victorian composers, but it appears that at heart he was a progressive. However, more than one attempt on his part to present the unfamiliar resulted in hostile reaction from press, public and choir alike. One case was his choice of Brahms's Schicksalslied (Song of destiny). The New Zealand Herald lamented the choice of 'this dull and funereal dirge', and some members of the choir's tenor and bass sections elected to join the audience rather than accept the challenge posed by this modern work.
Schmitt was naturalised in April 1888. His interests apart from music were centred mainly on the military. He held the rank of honorary captain in two volunteer units, and was a captain in the New Zealand Militia. He was honorary aide-de-camp and musical director to two New Zealand governors, and was also an active member of the Auckland Society of Arts.
His students and the choral society recorded their appreciation of his courtesy and his kind and genial manner, although in 1899 he was censured by the society's committee for an alleged lack of courtesy during rehearsals. By this time Schmitt was in poor health. During his last performance of Messiah, just three months before his death, he had frequently to hand over the baton to his assistant.
Schmitt died at Clevedon on 22 March 1900, survived by his wife and two children. His musical lineage, the scope of his early training, the prestigious posts and awards he held, and laudatory press reviews attest to his standing as a musician. The New Zealand Observer claimed that 'as a musical conductor he was admittedly without a rival in New Zealand'. His incumbency through its formative years helped ensure the survival of what was to become the Conservatorium of Music at the University of Auckland.