Ballynahinch started slow and gave the ball away twice on the edge of their own box within the first two minutes.
After the third time, Saintfield scored a header from a similar mistake after five minutes.
United never settled and couldn’t find a rhythm and soon found themselves two down but it remained salvageable to half time.
After the break United’s performances didn’t improve looking sluggish on and off the ball despite good performances previously in the season.
Saintfield compounded the home team’s misery by scoring more.
Despite a triple substitution after an hour, United’s Sky Blues failed to make any real dent in the deficit and were well defeated in the end by a team who clearly worked harder and wanted it more.
Saintfield United progressed to the next round of the Border Cup.
Ballynahinch Camera Club made their debut at the club and took some spectacular pictures.
Line up: Stefan Graham (C), Thomas Rodgers (Tyler Dickson), Stuart McMullan, Scott Brashaw, Chris Coleman, Gavin Crawford, Ronnie Johnston, Greg Hall, Richard Annett (Glenn Bennett), Lee Maxwell, Jack Patterson (Cameron McKissick).