Christiansen showed exceptional strength at an early age. In 1971, he became the first junior high school student to win the National High School Championship. He went on to win three invitational U.S. Junior Championships in 1973, 1974, and 1975. In 1977, at age 21, he became a Grandmaster without first having been an International Master, an accomplishment shared by only a handful of others. Christiansen tied for first place with Anatoly Karpov at Linares 1981. He won the 2001 Canadian Open Chess Championship. He also won Curaçao 2008.

Christiansen played on the United States Olympiad teams of 1980, 1982, 1984, 1986, 1988, 1990, 1992, 1996 and 2002.

Christiansen describes his playing style as "aggressive-tactical" and he lists his favorite opening as the Sämisch King's Indian. He is the author of two popular chess books that showcase his aggressive style: Storming the Barricades (2000) and Rocking the Ramparts (2004).

Christiansen is also renowned for his quick wit and humor, as well as his enthusiasm for teaching students. He is also one of the most prolific internet chess players, having played tens of thousands of games online on ICC (Internet Chess Club).

He is currently ranked 11th in America and 262nd in the world with a FIDE rating of 2578. FIDE Online. Individual world chess ranking