Kelly has been appointed defence coach of Texas rugby union club Houston Sabercats.
A one-time Super League coach of the year, he had two spells in charge of Rams two decades apart, winning the Northern Ford Premiership Grand Final in 2000 and keeping them in the Betfred Championship in each of the past two seasons.
He also coached Widnes Vikings when they were promoted to Super League and was in charge of Wales and Leigh Centurions. He left Rams at the end of the 2018 season for personal reasons and also because of concern over the future playing budget.
In rugby union, Kelly worked in Ireland and with the Namibia and Romania national teams.
He described his new role in Houston as an “adventure”.
He said: “It is only the second year of Major League Rugby and it seems to be growing year on year.
“There’s two more clubs coming in next year, including Toronto and three more in 2020.
“It’s exciting to be joining it in its first year when it’s just getting started.
“I am sure the quality and prestige of it will grow every year.”
Kelly admitted he is “definitely sorry to be leaving rugby league”.
He added: “It is my first sport, it is what I know, but you can’t stay in rugby league if there’s no job.
“I know I stepped down at Dewsbury, but at this stage in my career it is a no-brainer having this opportunity to go to Houston.
“It is exciting. In our first three games we play Austin – which is a local derby – and then San Diego, on the west coast and New York, which is on the east.
“It has got a different ring to it than Batley at Batley or Swinton at Swinton.”
Houston, who are preparing to move into a new, purpose-built stadium, finished last of the MLR’s seven teams this year, but have begun preparations for the new campaign with two big wins in pre-season matches.
Kelly will link up with his new club in January ahead of the season starting the following month.