Boyd scored eight, including a pair of 3-pointers, during a 13-2 run to open the game and the Kangaroos never trailed.
Seattle (6-9, 0-1) trailed 38-26 at halftime and Manroop Clair's 3-pointer pulled the Redhawks within eight with 14:40 to play. Boyd answered with a jumper, and later hit his final 3, as UMKC scored 10 in a row to push its lead to 18 with 10:31 left.
The 'Roos made 23 of 30 free throws from there to seal it. They finished 30 of 43 from the foul line - both season highs.
Clair led Seattle with 20 points. Brendan Westendorf added 17.