In September 1924.
Buddenbrooks, the first of Thomas Mann's works to appear in English, is published in a translation by the American Helen T. Lowe-Porter. The original German appeared in 1901.
Buddenbrooks, volume 1 of 2 by Thomas Mann at PG:
https://gutenberg.org/ebooks/72961
Volume 2 of 2:
https://gutenberg.org/ebooks/72962