Mason Oberg scored 19 points to go with nine rebounds, six assists and six steals as the Titans won the non-league game in Seattle.
Union held Lake Washington to single-digit scoring in the final three quarters after leading by just seven points after the first quarter.
“I was really happy with our defensive effort,” Union coach Mike Cranston said. “It was a good win for a road trip to Seattle this early in the season.”
Mackenzie Lewis had 12 points, Kaneyl Carpenter 10 points, Marina Morningstar 11 rebounds, and Rebekeah Barney six steals.