Owen Callaghan, Jared Rance and Dominic Shepherd were on target for the visitors, who made the long trip with a bare 11 owing to injuries and unavailability.
Lancers boss Richard Whittington said: “It was a must-win game for us so to come away with such a convincing performance and result is very pleasing.
“We controlled it from start to finish and never looked in danger and it is credit to the players that they dealt with the pressure and produced the goods.”
Selsey fielded a young side following the recent exodus of most of their side and they found themselves behind after just three minutes.
Rance did well to cross from the right and Callaghan arrived late in the box to head home.
Lancing thought they should have had a penalty midway through the first half when Jonno Melia went down and they had to wait until 20 minutes from time before they added their second.
Orran Guile did well to steal the ball from the home defence and Rance planted a shot from the edge of the box in off a post.
Lancing sealed the points with the best goal of the game five minutes later when Brad Bant cleverly stepped over a cross from Rance for Shepherd to cooly slot home.
Lancers dropped back into the bottom three on Tuesday, after Hailsham won 2-0 away to Selsey.
Lancers host Dorking Wanderers on Saturday (3pm), before they entertain local rivals Worthing United on Easter Monday (11am).
LANCING: Shaw, Fenton, Mills, Davis, Sharman; Bant, Callaghan, Melia, Rance, Guile; Shepherd. Subs: Gurney, Stuart.