"Hark! The Herald Angels Sing" was written in 1739 by Charles Wesley, the brother of the founder of the Methodist church, John Wesley. Though Welsey did not mind people reprinting and using his song, he did not want any words changed and he went so far as to put that in writing. His friend and co-worker, George Whitfield, ignored that request and changed the wording of the first two lines from "Hark! how all the welkin rings / Glory to the King of Kings" to what we use today.

The original music played with "Hark! The Herald Angels Sing" was also different from the current version, being slow and solemn, as that was what Wesley wanted and that was how the song was played for 101 years. In a completely unrelated event, in 1840, composer Felix Mendelssohn wrote a song to commemorate the invention of the printing press. He said that his music could be used for other works, with the exception that it was not to be used for sacred music. Fifteen years later an organist named William H. Cummings ignored that request and adopted the music from the second chorus of Mendelssohn's song for the carol. He also organized the song into the ten-line stanzas that are sung today. The final version was published in 1856.

--christianmusic.about.com