Born in Austria, Arnold became an American citizens 40 years ago and on he described the moment as the 'proudest' moment in his life.

"On this day 40 years ago, I became an American citizen,"  the former governor of California wrote on Instagram.

Arnold further said: "It is one of the proudest days of my life. I owe everything to America. Born in Austria, Made in America!"

He served as the 38th governor of California from 2003 to 2011 and was among Time magazine's 100 most influential people in the world in 2004 and 2007.

After playing the title character in the science fiction film The Terminator (1984), he starred in Terminator 2: Judgment Day (1991) and three other sequels. His other successful action films included Commando (1985), The Running Man (1987), Predator (1987), Total Recall (1990), and True Lies (1994), in addition to comedy films such as Twins (1988), Kindergarten Cop (1990) and Jingle All the Way (1996).