Williams has signed a two-year deal with the current Ranfurly Shield holders for the 2015 and 2016 seasons.
In December later year, Williams confirmed he was returning to rugby after announcing a two-year deal with the Chiefs, a franchise he won the 2012 Super Rugby title with.
Steelers coach Tana Umaga is rapt to be reunited with Williams, after coaching the league convert at French club Toulon for two seasons from 2008-2010.
“Sonny is an outstanding man, whose work ethic, ability and motivation are exceptional. He will be a great role model to our younger players and an inspiration to the young people of South Auckland.”
The powerful midfielder is looking forward to joining Counties Manukau.
“I like what is happening at Counties. I know a few boys in the side and enjoyed working with Tana,” Williams said.
“I have enjoyed my time in Australia and at the end of the season I will be ready for next chapter of my career back in New Zealand.”
Williams is expected to return to New Zealand once he has finished his commitments with his NRL team, the Sydney Roosters and will play his Super Rugby for the Chiefs for the next two seasons.