Lawler, 26, made his debut for Robins in 2015 and was their longest-serving player.
Hull KR confirmed in May he had turned down a new, improved deal - which included a testimonial - and would be leaving at the end of this season.
“I’m really excited,” Lawler said of his move to Castleford.
“I’ve been at Hull KR for six or seven years and loved my time.
"To now come to Castleford, I’m really looking forward to it, linking up with Radders [coach Lee Radford].”
Tigers finished seventh in Betfred Super League this year, one place below Hull KR.
But Lawler insisted: “I think the signings we’ve made are exciting.
“I’ve spoken to a couple of boys who’ve worked under Lee and there has been nothing but positive things.
“The style of play he plays suits me too, so I’m looking forward to working with him.”
Lawler’s final game for Hull KR was a defeat by Huddersfield Giants at the end of August, when he suffered a dislocated thumb.
“I think this year has been the best year I’ve been at KR, rugby-wise, but I’m looking forward to a new challenge with Cas,” he added.
“I’m sure we can do good things.”
Lawler is among six new faces confirmed in Tigers’ 2022 squad.
Of what he can add to his new team, he said: “I think I bring a lot of work rate.
“I do the little things that nobody really sees to help the team out.
“It helps lay the platform for the backs and halves.
“I can’t wait to play in front of the Cas fans, wear the jersey with pride and passion and do my bit for them.”
Radford, who has taken over from Daryl Powell as Castleford coach, believes Lawler will fit well into the club’s culture.
He said: “You look at the honesty and hard work you get from the players here at Castleford Tigers, players like Nathan Massey and what he brings to the table.
“George is exactly the same.
“Hopefully now you can see the theme of what we want the team to look like with these types of players in there.
“I can’t wait to work with George.”