In what was a physical encounter, the visitors had Chris Laidlaw to thank for their hard-fought victory - the stand-off scoring 18 points as the Borderers secured the win.
Haddington's only points came through a brilliantly-created try by winger James Ferguson and Nick Bell's conversion, despite large periods of pressure in the first half.
It was a match that took a while to get started, with the first score taking over half an hour to arrive, but it was Jed who struck first, as centre Monty Mitchell crashed his way through the home defence to score under the posts. (0-7)
Just before the break though, we responded with a try of our own, as back-row Connor Bolton worked the ball out of the contact superbly to find winger Ferguson, and last season's joint-top try scorer darted over. Bell's conversion tied the game at the break as both sides battled tricky conditions.
Half-time: Haddington 7 - 7 Jed-Forest
After the resumption, the visitors showed the class that saw them beat the much-fancied Howe of Fife last weekend, as they mounted concerted periods of pressure on the Haddington line.
Only fierce defence from Mamea, McCutcheon and Martin kept the Border men at bay.
That was until Laidlaw's late double, as the mercurial stand-off crossed twice inside the last ten minutes to ensure the points would be travelling to the Riverside and that Haddington left their opening home match of the season empty handed.
Full-time: Haddington 7 - 28 Jed-Forest
There were undoubted plus points to come from the match, with the home debut of Mamea and a first outing of the season for James Ferguson clear highlights.
The return of Ross Martin to the hallowed Neilson Park turf was also a big boost, while another former Heriot's man, Sandy McCutcheon, also showed up well in the back row.
The squad will now re-group ahead of another tricky challenge next weekend, as they travel to Livingston.