In 2003 (we don't have the exact date) mountain biking legend John Stamstad ran the trail self-supported in 24h01m. His trip was reported in the May 2004 issue of UltraRunning Magazine.
Mitchell Rossman provided a bit more detail in 2019:
"John did his FKT the second weekend of September 2003. John was entered in the 2003 Plain 100. The race got canceled three days before the scheduled start because of forest fires. So John, all worked up with no place to run, went and ran his Wonderland Trail FKT while many of us did John Varner’s last minute Cure For The Plain.
When I saw John’s account posted on line soon thereafter, i got all worked up and I did my own unsupported run of the Wonderland Trail in September 2004.
I seem to recall that John wrote that he heard of only one other person doing the Wonderland Trail in about 28 hours before him."