Williams was named as a replacement for Mitsubishi Dynaboars clash with Kubota Spears and the wing wizard came off the bench to grab the match-winning try for his new club.
But the impressive debut came at a cost for Williams, who dislocated his thumb in the process.
Williams penned a six-month deal with the Dynaboars at the beginning of the month despite earlier announcing his retirement from the game.
The 2008 IRB Player of the Year quit international rugby following the 24-18 defeat to Australia in December, which was billed 'Shane's farewell.
Williams signed off from Wales duty with a spectacular late try, to take his international tally to 58 in 87 Test appearances.
He then bode farewell to the Millennium Stadium, playing for the Barbarians against Wales earlier this month.