Originally recorded by Evie Sands (1965).
Hit versions by The Hollies (US #42/UK #2 1966), Linda Ronstadt (US #31 1980).
From the wiki: “Co-writer Chip Taylor (co-writer ‘Wild Thing‘, ‘Angel of the Morning‘, ‘Try (Just a Little Bit Harder)‘) is the brother of actor Jon Voight. The song was first recorded by blue-eyed soul songstress Evie Sands, and was locally popular in the US [NYC] in 1965.
“‘I Can’t Let Go’ was The Hollies’ first 1966 single, and the final single to feature the band’s original bassist Eric Haydock. It went to #2 in the UK but only #42 on the Billboard Hot 100 in the US. (The Hollies’ version got an enthusiastic thumbs up from Paul McCartney, who thought Graham Nash’s soaring tenor during the chorus was a trumpet.)
“Linda Ronstadt released a successful version in 1980, from her Platinum-certified album Mad Love. It reached #31 on the Billboard Hot 100.”
The Hollies, “I Can’t Let Go” (1966):
Linda Ronstadt, “I Can’t Let Go” (1980):