Anyways, the original song "At Last" was written in 1941 by Mack Gordon and Harry Warren for the musical Orchestra Wives, which starred George Montgomery and Ann Rutherford. For the film, it was recorded by Glenn Miller and his orchestra and sung by Ray Erberle and Pat Friday. It reached number 14 on the charts in 1942 and became a standard. Many other singers, such as Nat King Cole, did covers of the song.
Here is a video of the original song: