The Star Wars actor said he was still a "proud Irishman", but the warmth from well-wishers in the US during such a difficult time prompted his decision.
Broadway star Natasha died in March following a skiing accident in Quebec, Canada. The actress died three days after a fall on a beginner's slope lesson in the resort of Mont Tremblant.
Speaking on chat show Good Morning America, the star said his two teenage sons Michael and Daniel were "doing good", five months after the tragic accident.
"To be honest, we are taking each day as it comes," he added.
Liam, born in born in Ballymena, County Antrim, said he was still getting "deeply touching" letters of condolences from people in the States.
"That's partly the reason why I've recently become an American citizen," he explained.
On the show to promote his new film Five Minutes Of Heaven, he also paid tribute to senator Ten Kennedy, who passed away this week, describing him as "a very special man."
"I met him a couple of times over the years, and he wrote my family a very, very beautiful, touching condolence letter when Natasha died," he revealed.
In his new film – which was shown on BBC Two in April, and has opened in selected US cinemas - he plays a former member of an outlawed Northern Ireland paramilitary group who is haunted by memories of murder.