There are a few mixed martial artists who competed at a very high level of a particular sport before focusing solely on MMA. Usually, they were either world class wrestlers or kickboxers. No one has made a successful transition from classical boxing to MMA. Probably because the money is still much better for fighters in boxing.
Wrestlers who were world class before fighting MMA: Randy Couture, Dan Henderson, Matt Lindland; (Chuck Liddell, Matt Hughes, and Josh Koscheck all competed in college)
Kickboxers who were world class before fighting MMA: Mark Hunt, Mirco "Cro-Cop" Flipovich, Semmy Schilt (although he might have found MMA before kickboxing), Duane "Bang" Ludwig (although he has had mixed success); I think Anderson and Vanderlei Silva were fairly successful muay thai fighters in Brazil.
You have some mixed martial artists who have fought some low level boxing matches and done well, including: Jens Pulver, Chris Lytle, Yves Edwards