The sides had met in the reverse fixture just two weeks ago with Grangemouth running out comfortable winners.
The Stags made a solid start and scored after just five minutes. Mark Christie crashed over for the opening score. Shannon was unable to add the extras.
The early score failed to settle the home side and they were guilty of giving away several penalties, allowing Harris to kick the ball into the Stags 22. From a lineout they brought their centre in on an angle to slice through and score a converted try.
On 20 minutes, the visitors added a second score as the winger squeezed in at the corner to extend the lead.
The Stags replied a few minutes later as the ball was moved out to Dempsey who ran round the cover to score. Shannon was wide with the kick. Once again Grangemouth seemed to relax and another missed tackle allowed Harris to break the line and score under the posts.
The last five minutes of the half saw the Stags finally exert some control to score two tries. First, the pack drove on with Keigan staying bound at the back to drive over for his first competitive try for the club. Shannon slotted the conversion. Then, with the last play of the half Dempsey collected, his second score. Shannon added the extras for a half time score of 24-19.
Grangemouth were straight back on the attack at the start of the second period and young winger Downie drew the last man before slipping the pass back to Chapman for the score. Shannon kicked the conversion.
Straight from kick-off, Binnie carried on before passing to Scott for the score. The conversion was wide.
Once agin the home side seemed to lose focus and the visitors to snatched their fourth try for a bonus point. On the hour mark Grangemouth added their seventh try. Dempsey dived over from short range and Shannon slotted the conversion.
Allan was given a yellow card at the end before Dempsey added another score for the final score of Stags 50-24.