Felix Mendelssohn did not compose "Hark, the Herald Angels Sing." The famous Christmas carol was actually written by Charles Wesley, a Methodist hymn writer, in 1739. The melody that is commonly associated with the carol today is attributed to Felix Mendelssohn, but it was not originally intended for this hymn.
Mendelssohn composed the melody as part of a cantata titled "Festgesang" or "Gutenberg Cantata" in 1840 to celebrate the invention of the printing press by Johannes Gutenberg. The melody was later adapted by English musician William H. Cummings, who paired it with Wesley's lyrics to create the version we know as "Hark, the Herald Angels Sing." This fusion of Wesley's words with Mendelssohn's melody became popular and is now one of the most beloved Christmas carols.