Scott was bidding to become the first player to complete a hat-trick of back-to-back wins at the tournament, but the world No 2 could not make up the necessary ground following a slow start in the first round.
Scott carded six birdies and two bogeys to record a four-under-par final round of 68 and finish on eight-under for the tournament – one shot behind Cullen, whose three-under-par 69 was enough to clinch victory at the Metropolitan Golf Club.
Cullen, 30, made his move with an eagle at the fourth hole before adding birdies at 11 and 12.??
Scott had three birdies on the back nine while another birdie putt at the last that could have brought him level with Cullen lipped out.
Cullen, who was third at start of play, wobbled with a bogey at 15 but sealed victory thanks to a superb bunker shot at the final hole which allowed him to make a winning par.
Scott was tied for second with Josh Younger and James Nitties, while overnight leader Paul Spargo was one of four – the others being Matthew Griffin, Aron Price and Adam Bland – to finish two shots off the winning score on seven-under.
Spargo was in the reckoning after three rounds but a one-over-par 73 on Sunday, which included three bogeys and just two birdies, cost him dear.
Michael Wright, who led after the first and second rounds, had a Sunday to forget as he recorded a triple bogey, a double bogey and five bogeys in an eight-over-par final round to finish 10 shots off the pace on one-over par overall.