He composed symphonies and song cycles on poems by Friedrich Hölderlin, Joseph von Eichendorff, Georg Trakl and Peter Härtling, among others.

Killmayer's first composition receiving attention was Lorca-Romanzen after Federico García Lorca, premiered at the Donaueschingen Festival.

[5] In 1954 he composed a Missa brevis, which was recorded and reviewed:Young (29) Munich-born Composer Wilhelm Killmayer's Missa Brevis ripples with exciting, shifting rhythms and rises skillfully to a colorful series of blasting choral climaxes occasionally more reminiscent of the bandstand than the choir.

[7] He composed other orchestral works such as Nachtgedanken (1973), and music for chamber orchestra, The woods so wilde (1970), Schumann in Endenich (1972) and Kindertage (1973).

[8] For the 20th anniversary of the Münchener Kammerorchester Killmayer composed in 1970 Fin al punto for string orchestra, which premiered in 1971, conducted by Hans Stadlmair.

[10]Interested in poetry and the voice, he composed Lieder, three cycles of Hölderlin-Lieder based on poems by Friedrich Hölderlin (1980s), also song cycles based on Joseph von Eichendorff (1991), Georg Trakl (1993 and 1996) and Peter Härtling (1993), and ballads such as Heinrich Heine's Ali Bey (2006) and Eduard Mörike's Der Feuerreiter (2007).