[3][2][6][1] He played the opening bars of Michael Tippett's King Priam at its Coventry premiere in 1962, conducting the whole work years later for English National Opera (ENO).

[3] Howarth's (unplanned) conducting debut was with the London Sinfonietta on tour in Italy in 1969;[2] his first operatic assignment was for The Rake's Progress for the Royal Northern College of Music at Sadler's Wells in December 1973.

[8] Following further work on the concert platform, Ligeti engaged him for the premiere of his Le Grand Macabre at the Royal Swedish Opera in Stockholm in 1978,[6] as well as in Hamburg and Paris.

[1] For the British stage premiere of Le Grand Macabre at the London Coliseum in 1982, Howarth wrote an analysis of the text and music for Opera magazine.

At Glyndebourne (and on tour with the company) he conducted Rossini's Il Barbiere di Siviglia and Verdi's Falstaff in 1981, Nigel Osborne's The Electrification of the Soviet Union in 1987 and 1988, and Birtwistle's The Second Mrs Kong in 1994 and The Last Supper in 2000 and 2001.

[3] He first appeared at the Proms in 1970 in a late-night concert of music by Mike Ratledge of the experimental rock band Soft Machine, Terry Riley and Tim Souster.

Composer Roy Newsome remarked that "Howarth's masterly rendition of Mussorgsky's Pictures at an Exhibition (1979) dwarfed all previous transcriptions".

[2][25] His discography includes Schoenberg's Pierrot lunaire with Cleo Laine and the Nash Ensemble,[26] and the suite from Stravinsky's The Soldier's Tale, Dumbarton Oaks and Octet for wind in 1974,[27] works by Brian Ferneyhough with the London Sinfonietta in 1978,[28] Copland's Appalachian Spring and Music for Movies with the London Sinfonietta in 1981,[29] music for brass by Paul Hindemith with the Philip Jones Brass Ensemble in 1981,[30] Birtwistle's Gawain with the orchestra and chorus of the Royal Opera House in 1996,[31] which won a Gramophone Award in 1996,[3] and Bliss with the forces of Opera Australia in 2015.

[32] A number of personal copies of works Howarth conducted (some including annotations) are catalogued[33] at the University of East Anglia's School of Music.

[2] With his son Patrick, Howarth wrote a book "which explains the brass band world", with chapters on the repertoire, and interviews with among others Derek Bourgeois and Major Peter Parkes.