Toronto – only formed four years ago – marked their debut in the top flight by scoring an early try, Liam Kay getting on the end of a grubber kick in the first of two games played at Headingley on Sunday.
Williams was on the bench at that stage and the New Zealander, who last played in the 13-man code in 2014, endured an inauspicious start following his introduction to proceedings, knocking on with his first touch.
Castleford recovered quickly after falling behind, Greg Eden, Grant Millington, Adam Milner and Michael Shenton all scoring tries before the break.