Co-written and first recorded by Ed Bruce (C&W #15 1975).
Other hit version by Waylon Jennings & Willie Nelson (US #42/C&W #1/CAN #1 1978).
From the wiki: “‘Mamas Don’t Let Your Babies Grow Up to Be Cowboys’ was first recorded in 1975 by Ed Bruce, written by him and his wife, Patsy Bruce. Bruce’s rendition of the song peaked at #15 on the Hot Country Singles charts in 1975.
“Waylon Jennings and Willie Nelson covered the song on their 1978 duet album Waylon & Willie. Unbeknownst to Bruce, Jennings had recorded it himself in 1977. Unfortunately, Waylon was unhappy with his recording. So he brought it to a session with Nelson, who added vocals to the existing recording. Jennings recalls, ‘Me and Willie were talking about recording again, and I said, ‘Willie, I cut this thing, but I ain’t sure about it.’ I said, ‘It don’t sound right, but it might be a great duet.’ {Even though my] whole record was finished, I just took [out] part of my voice and put his on.’
“The Jennings/Nelson cover recording peaked at #1 on the Hot Country Singles chart in March 1978, spending four weeks topping the chart while also crossing-over to the Billboard Hot 100. In 1979, the song won the Grammy Award for Best Country Performance by a Duo or Group with Vocal. Members of the Western Writers of America have also chosen ‘Mamas Don’t Let Your Babies Grow Up to Be Cowboys’ as one of the Top 100 Western songs of all time.”
Waylon Jennings & Willie Nelson, “Mamas Don’t Let Your Babies Grow Up to Be Cowboys” (1977):