Ellis was 3 of 7 from the field -- all 3 makes coming from 3-point range -- and knocked down a game-high six free throws.
Army (3-1) took a 31-30 lead into halftime after Kyle Wilson made a layup as time expired in the first half.
The Black Knights opened the second half with eight unanswered points to go ahead 39-30. St. Francis cut the deficit to 45-44 with 10:06 left but Army held the Terriers to just one point over the next six minutes to pull away.
Army outshot St. Francis (1-2) 42.9 percent (24 of 56) to 33.9 percent (20 of 59) and got 35 points off the bench compared to just 10 for the Terriers.
Jalen Cannon led St. Francis with 23 points and 10 rebounds.
Nebraska guard Andrew White, who averaged 16.6 points and 5.9 rebounds per game last season, will transfer from the program and is eligible immediately. What else is on Dickie V's mind this week?
The Cavaliers bring in a top-10 class of four incoming freshmen, so the Class of 2017 will be a small one. Coach Tony Bennett is aiming high.
College basketball coaches across the country will hit the recruiting trail hard in July to target the Class of 2017's top prospects. Which players are the nation's top programs targeting?