I found this on "ethnicelebs":
What Race or Ethnicity?: German or Norwegian
The American wrestler and mixed martial artist was born in Webster, South Dakota and attended the University of Minnesota where he competed as an amateur wrestler. Along with his WWE stardom, Lesnar is a decorated NCAA wrestler and he became the undisputed UFC heavyweight champion during UFC 100 on July 11th, 2009. Lesnar’s ethnicity has many intrigued as his enormous physique leaves one wondering what race of people could produce such a freak of nature. The UFC didn’t even have gloves that would fit his enormous hands.
However finding out his ethnicity has been challenging and only discussion on various forums can been found when searching for his heritage. Most people seem to believe he is of either German or Swedish descent which may partially explain his very pale complexion and almost white hair. However in South Dakota it is indicated on the 2005 census that the largest ethnic group is German making up 40% followed by Norwegian at 15%. Swedish don’t make up a significant proportion in South Dakota but account for about 10% in neighboring Minnesota where Lesner went to college. With this I’d tend to think he is of German heritage.