The win guarantees the Kangaroos (10-18, 7-8 Western Athletic Conference) at least a No. 4 seed in the WAC tournament March 13-15 in Las Vegas.
UMKC led 13-7 on a layup by Harrison, but Seattle pulled ahead 31-26 on free throws by Shore Adenekan and led 31-29 at halftime.
UMKC used a 7-2 run to take a 36-33 second-half lead, then retook the lead 45-42 on a 3 by Williams, then pulled ahead by as many as eight. Seattle pulled to within three points on a trey by Jarell Flora with 22 seconds to play, but UMKC's John Burke iced the win with a dunk.
Nelson Kirksey scored 13 for UMKC and Isaac Kreuer 10.
Flora led the Redhawks (13-15, 5-10) with 19 and Emmanuel Chibuoqwu added 12.