All-America forward Chris Braier had 26 points and pulled down a career-high 24 rebounds for the sixth-ranked Vikings, who improved to 11-0 and 5-0 in the MWC. Braier had attained his double-double with seven minutes left in the first half.
John Grotberg led Grinnell (8-3, 4-1) with 31 points.
The Vikings withstood a Grinnell rally in the first half and then kept the Pioneers at arm's length for much of the second half. Lawrence led 39-24 with 10:16, but the Pioneers rallied to tie the game at 59-59 at the half.
Lawrence went on an 18-4 run early in the second half and pulled out to an 83-69 lead on Ben Rosenblatt's layup with 11:23 remaining.
Grinnell cut the lead to nine points three different times but never got closer than that.
The Vikings, who committed only 21 turnovers against Grinnell's pressure while forcing 15 of their own, made all 44 of their field goals in the paint and added 37 free throws.
It was a night of career highs for the Vikings. In addition to MacGillis and Braier, freshman guard Ryan Kroeger scored a career-high 20 points and handed out a game-high nine assists. Sophomore guard Erik Megow, who had scored only five points thus far in his career, hit 5 of 6 shots and all six of his free throws on his way to a career-high 16 points.
The Vikings shot only 69.8 percent (44-63) against Grinnell's "system" but did convert on 37 of 52 free throws. The Pioneers were 37 of 97 from the field (38.1 percent), including 18 of 51 (35.1 percent) on 3-pointers.
Lawrence returns to action at home on Saturday when it hosts Lake Forest College.