Australia 's Robertson collected the £100,000 top prize at the Ricoh Arena in Coventry after getting the better of Northern Ireland's Allen in a battle of two left-handers.
Former World, UK and Masters champion Robertson has had a quiet spell since winning the Wuxi Classic at the start of last season, his only other title coming at the Gdynia Open in February.
But the 33-year-old looked close to his best in winning four matches this week to win the event for the first time. World number three Robertson took a very cautious opening frame with a run of 36 then Allen knocked in a brilliant 103 to level.
Robertson then surged ahead as breaks of 53, 114 and 69 helped put him 4-1 up. The next four frames were shared, Robertson making a 71 in the ninth to lead 6-3 at the end of the first session. As the evening session got underway, Robertson picked up where he left off as runs of 60 and 40 helped extend his lead to 8-3. Allen, who grabbed the 16th and last place in this event by winning last week's Bulgarian Open, needed a response and pounced on a mistake from his opponent to clear from the last red in frame 12. And when he fired in a break of 112 for 8-5, the fight back looked on. But a scrappy 52-minute 14th frame proved to be crucial, as a safety error from Allen allowed his opponent to clear from blue to black to move one away from the title. It came down to the colours again as the next frame was equally untidy. A miss on the black from world number 11 Allen allowed Robertson to clear from yellow to blue to seal the victory.