He was best known in his lifetime as a conductor, and it wasn't until the middle of the 20th century that his music became more well known. This was partly due to his music being very modern (for the times), complex, most of the symphonies need a very large orchestra, and the works are lengthy. It also didn't help his cause when his music was banned during the Nazi era in Europe because of his Jewish heritage.
Composing for Mahler remained a part-time activity for most of his life, undertaken during the opera and concert off-season. Later in life he would do his composing in a small hut that he had built by the lake and away from his main house.
The 1st Symphony was completed in 1888. He originally called it a two-part tone poem but after the premiere he made extensive revisions and called it his first symphony. One of the decisions he made was to reduce it to a more traditional 4 movement symphony when he eliminated one of the middle movements. The symphony calls for a very large orchestra, over 100 players.
Mahler was greatly influenced by a book of German folk poems called Des Knaben Wunderhorn, or The Youth's Magic Horn. He set some of these poems to music throughout his life beginning with four songs that he called Lieder eines fahrended Gesellen, or Songs of a Wayfarer. The opening movement of the first symphony uses the music of one of these songs, Ging heut' Morgen übers Feld, or I Went This morning Over The Field.
Ländler, while the third movement shows Mahler at his morose best. The opening of the third movement is the folksong Frere Jaque (Bruder Martin in German) played in the minor key as a funeral march. Mahler's inspiration for this was a visual he had of a group of animals of all description marching with the corpse of a hunter for burial. This movement also uses one of the songs from Lieder eines fahrended Gesellen. The final movement is the most complex as some of the previously heard music is brought back and interspersed with new themes.
Mahler once said, "A symphony must be like the world. It must contain everything." His music is big in every sense of the word. Big in expression, forces used, length and complexity. And each one of his symphonies is like a world unto itself.