After Michael Bolden's 3 with 14:46 remaining pulled Cal Poly (9-15, 5-6 Big West) within one, Lamb scored eight straight, including back-to-back 3-pointers, to open up a 47-38 LBSU lead.
Cal Poly trimmed its deficit to three just 1:12 later but would get no closer.
Long Beach State (11-14, 7-4) caught fire down the stretch, making 16 of 24 field goals (66.7 percent) after halftime. The 49ers outscored the Mustangs 40-24 in the paint for the game.
Kyle Odister, who finished with 11 points, was the only player in double figures for Cal Poly, which never fully recovered from a cold first half in which it shot 30 percent, making just 2 of 9 from 3-point range.
Dan Jennings finished with 12 points and nine rebounds in the win.