McColl's move to Headingley had always seemed likely following his departure from the Knights, however Carnegie have now officially confirmed the two-year deal.
The Scottish back scored 13 tries in 27 competitions last season and he is relishing the chance to move to Leeds.
He said: "I had a fantastic time at Doncaster and I can not thank everyone at the Knights enough for the support they have given me.
"However, I am looking forward to being part of the full-time set-up here at Leeds Carnegie and I am excited to be working with the new coaching team here at the club.
"It has been a bit like your first day at school with the start of pre-season and you are desperately trying not to forget anyone's name but I am sure we will all get to know each other.
"It is going to be a long season but with the way the format is set up now with the play-offs we have got time to gel as a squad and make sure that we are ready for the play-offs."
Meanwhile 24-year-old Paul has agreed a 12-month extension to his deal, with the homegrown star excited about the new season.
"Despite relegation last season, the belief within the squad is really high. I am looking forward to working with Diccon Edwards again," said Paul.
"He is someone who is very passionate and enthusiastic about the club and that will be important."
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