"I'm an Old Cowhand (From the Rio Grande)" is a comic song written by Johnny Mercer for the Paramount Pictures release Rhythm on the Range and sung by its star, Bing Crosby. The Crosby commercial recording was made on July 17, 1936, with Jimmy Dorsey & his Orchestra for Decca Records. It was a huge hit in 1936, reaching the No. 2 spot in the charts of the day, and it greatly furthered Mercer's career. Crosby recorded the song again in 1954 for his album Bing: A Musical Autobiography.
Members of the Western Writers of America chose it as one of the Top 100 Western songs of all time.

Background
Mercer and his wife were driving across the US en route to his hometown, Savannah, Georgia, after having apparently failed to succeed in Hollywood. Mercer was amused by the sight of cowboys, with spurs and ten-gallon hats, driving cars and trucks instead of riding horses. Singing cowboys were popular in films and on the radio then, and within 15 minutes, writing on the back of an envelope, Mercer transferred the image he was seeing into a song whose satirical lyrics vented some of his own bitter frustration with Hollywood.

The lyrics, about a 20th-century cowboy who has little in common with the cowpunchers of old, have been included in some anthologies of light verse.
Summarize in one sentence this article about a famous song.
I'm an Old Cowhand (From the Rio Grande), written by Johnny Mercer and recorded by Bing Crosby, is popular song from 1936 that satirizes cowboy culture of the time.