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.
Peter Mata'utia crossed in the second half for the Tigers before a long-range interception try for Toronto's Hakim Miloudi, who celebrated by doing a somersault to ground the ball over the line.
The debut pic.twitter.com/ygyo8NNV8R
— Toronto Wolfpack (@TOwolfpack) February 2, 2020
In the final fixture in the opening round of the new season, Hull FC eased to a 30-4 victory over Leeds Rhinos in the pouring rain.
Ratu Naulago grabbed two of Hull’s five tries, Carlos Tuimavave, Josh Bowden and Jamie Shaul also going over as the visitors produced an impressive performance.
Ash Handley touched down for an unconverted try for Leeds, but the Rhinos – who flirted with the prospect of relegation last season before eventually finishing eighth – were soundly beaten.