The 31-year-old departed English Championship side Queens Park Rangers in April following the expiry of his contract at the end of the 2017-18 season, thus, bringing to an end his six-and-a-half-year stay at the Loftus Road Stadium.
Onuoha started his career with City youth academy before he progressed to the first team and left for QPR in 2012.
The defender revealed that the desire for a new challenge motivated him to sign for the Claret and Cobalt.
"I have always been someone that wanted to go an experience something a bit different," Onuoha told BBC.
"I spoke to a few Championship clubs quite close to here [his home in Manchester], one in the Premier League, but it didn't quite feel right and I didn't want to retire thinking 'I wish I had done this'."
Onuoha could make his debut for Real Salt Lake when they clash with Minnesota United on Sunday at Rio Tinto Stadium.