There were also wins for Stirling County, last year’s runners-up, and Jed-Forest, against Glasgow Hawks and Selkirk respectively.
At Meggetland, a break by Paddy Anderson caused the Boroughmuir defence problems and the Melrose forwards rumbled over for the opening try, converted by Jason Baggott.
Muir got back into things with penalties in the 18th and 28th minutes by Gavin Parker to make it 7-6. An interception try by Kerr Gossman put the home side ahead. A penalty by Struan Hutchison for Melrose pulled things back to 11-10 at the break.
The hosts got off to a dream start to the second half, a penalty being followed up by a converted try to make it 21-10.
Melrose got it back to 21-17 through a converted Anderson try, but with 15 minutes left, were trailing 28-22. A try from Baggott, converted by Hutchison, put them a point up and a converted Iain Moody effort at the death sealed it.
At Bridgehaugh, Stirling County scored four second-half tries in a 27-12 win over Glasgow Hawks.
Jed-Forest beat Selkirk 18-10 at Riverside Park.