Stewie's owner, Robin Hendrickson, said the pet died on Monday evening after having cancer for a year.
The cat, of Reno, Nevada, was awarded the record, measured from the tip of the nose to the end of the tail, by the Guinness Book of Records in 2010.
Stewie was a certified therapy animal that frequently visited a local senior centre near his home.
Ms Hendrickson said: "Stewie was always very social and loved meeting new people. He has touched many lives, and for that I am grateful."
He was also the record-holder for world's longest cat tail.
The world's longest domestic cat, a 48.5in (1.23m) Maine Coon named Stewie, has died at the age of eight.