At the Metropolitan Opera in New York, Walker has led performances of Gluck's Orfeo ed Euridice, The Barber of Seville, The Magic Flute, and Les Pêcheurs de Perles, as well as the National Council Auditions Grand Finals Concert in 2016.
For English National Opera he has conducted Lucia di Lammermoor and The Tales of Hoffmann.
In 2008 he conducted the US premiere of William Walton's opera Troilus and Cressida at the Opera Theater of St Louis, and in the 2018–19 season for Washington Concert Opera he conducted the US premiere of Gounod's Sapho (starring Kate Lindsay) and the first performance in the US since 1835 of Rossini's Zelmira (with Silva tro SantaFé, Lawrence Brownlee, and Vivica Genaux).
In 2022 Walker conducted the world premiere of In a Grove, by Christopher Cerrone, for Pittsburgh Opera.
He has conducted the Sydney, Melbourne, Queensland, Adelaide, Tasmanian, and West Australian Symphony Orchestras, the Australian Chamber Orchestra, the BBC National Orchestra of Wales, Orchestre Colonne in Paris, Thessaloniki State Symphony, and made his debut with the Mozarteum Orchester in Salzburg in Jan 2019 with a program of Berlioz and Tchaikovsky.