Lamar's Kade Harrington carried 36 times for 282 and five touchdowns - all school records - but had a key fumble in the fourth quarter which led to a 95-yard return touchdown by Adam Jones to put Northwestern State up by 20.
Harrington scored twice in the fourth, on 2- and 3-yard runs, but Llorens' 1-yard run kept the Cardinals at bay.
Northwestern State (1-5, 1-3 Southland) jumped out quickly, taking a 14-0 lead after one quarter on Daniel Taylor's 12-yard run and Llorens' 53-yard scamper.
Harrington scored twice in the second - on runs of 79 and 5 yards - but Stephen Rivers completed touchdown passes to Shakeir Ryan and Zach White and the Demons led 31-14 at intermission. Rivers finished 15 of 21 for 197 yards.
Lamar fell to 3-3, 2-2 in conference.