Returning for his first action of the season King - who had been sidelined since February 2009 with a hamstring injury - crossed twice and had a hand in many more in the nine-try fest.
And Scarlets coach Nigel Davies was not shy in his praise for the returning Kiwi.
"It was never going to be easy for Regan coming back from such a bad injury, but he is starting to fire again," said Davies.
"He's so dangerous. Defenders don't know what he's going to do next and we saw out there what he is capable of. It was great to see him coming back to somewhere near his form."