Ashton, who suffered a major form lapse during this season's RBS 6 Nations Championship, touched down 17 minutes from time to confirm a Twickenham last-four appointment with Toulon or Leicester on April 28.
England fly-half Owen Farrell kicked 17 points from five penalties and a conversion of flanker Will Fraser's first-half try as a crowd of just under 38,000 saw Saracens never seriously troubled.
Ulster had fleeting moments but they failed to consistently trouble Saracens defensively after the home team took control from an early stage.
Scrum-half Ruan Pienaar booted three penalties for Ulster and converted flanker Iain Henderson's late try, yet they never threatened a repeat of last season's Heineken Cup heroics that saw them reach the final.
A concerted spell of Saracens pressure resulted in Fraser powering over, although it required several reviews by television match official Eric Gauzins before the try was awarded.
Victory was sealed when prop Rhys Gill secured possession in midfield and Ashton ran around two defenders to claim a try that he celebrated with a trademark dive.
Tommy Bowe featured off the bench late on but Ulster were already down and out, well beaten by a team whose superior organisation and collective excellence made it a one-sided affair.
Henderson had the final say, posting a try that Pienaar converted, but it was Saracens' day and they now await the outcome of events in southern France on Sunday.
- Sports & Recreation
- Ruan Pienaar