The Mavs head into their eight-game homestand riding a 4-1 stretch that includes wins over Portland, Indiana and now Oklahoma City, though the Thunder were without star guard Russell Westbrook in Sunday's matchup.
Jose Calderon took advantage of Westbrook's absence with 16 points, seven assists and five rebounds for Dallas.
Kevin Durant poured in 30 points for the Thunder, who gave Westbrook a rest having won five of their previous seven games.
Dallas scored eight of the final 10 points in the first quarter to take a 29-23 lead and stayed in front the rest of the way.
Marion went for 10 points in the second quarter and the Mavs held a 59-48 lead at the break thanks to a 51.2 percent shooting effort in the half.
Deep makes by Nowitzki and Calderon opened a one-sided third quarter that saw the margin stretch to 21, 91-70, on Brandan Wright's alley-oop in the final minute.
Durant sat the final 10 minutes with the outcome no longer in doubt.