The first goal came from a free kick given just outside the box in the 17th minute when Uilliams' shot sailed into the net.
Khelemba Meitei scored the second goal as he headed home a corner kick at the 37th minute of the match.
After the breather, Lajong players upped the ante and Dempo's Lenny Rodrigues pulled down Uilliams inside the box. Cornell Glen made no mistake in converting the spot kick at the 50th minute.
Dempo managed to find a few attacks but the home side were never in much danger. The visitors went down to 10 men after Nirmal Chettri picked up his second yellow card following a foul.