Day and Williams were together for just six events, starting in June at the U.S. Open. In those events, Day recorded just one top-10 finish while missing the cut twice.
After not making the cut at The Northern Trust this weekend, Day decided to move on from Williams.
Williams famously served as Tiger Woods' caddy from 1999 to 2011, and the 15-time major winner was the top-ranked golfer for the majority of Williams' tenure.
After parting with Woods in 2011, Williams began to caddie for Adam Scott. The two found immediate success, with Scott winning the 2011 WGC-Bridgestone Invitational, his first career World Golf Championship.
Day hasn't won a major since the 2015 PGA Championship, and his last PGA Tour win came in May 2018 at the Wells Fargo Championship.