The combined firm, which will operate under the Morton Fraser name, is projecting turnover in the region of £17 million with 41 partners and a staff of more than 250.
Morton Fraser, which can trace its history back to 1829, has headquarters in Edinburgh and operations in Glasgow and London.
Macdonalds, with its main base in Glasgow, employs a total of 11 partners and 47 staff and has a large debt recovery and litigation practice working for banks and financial institutions, as well as clients in the house-building and charity sectors.
The merger means Morton Fraser, headed by chief executive Duncan Murray and chairman Linda Urquhart, will significantly expand its presence in Scotland's largest city, with staff numbers growing to more than 70 and Glasgow-based partners at 13.
The deal is expected to formally be completed before the end of March next year.
Mr Murray said: "Macdonalds's practice enables us to both complement and expand our presence in Glasgow and the Macdonalds reputation and ethos make this an attractive cultural fit. Both firms are excited by the move which will see the combined firm ideally placed to provide our client base with increased specialist expertise.
"The merger is in line with our long-term strategic objective to build our presence in Glasgow and we have already received very
positive client feedback," he added.
In June this year Mr Murray predicted further mergers in the sector as he said the firm expected a slight drop in turnover from £14.7 million to £14.5m, but said profits would rise 7% from £3.6m to £3.9m thanks to a tight control on costs.
Macdonalds partner Robert Swindell will join the Morton Fraser managing board. He said: "This merger provides an excellent opportunity to offer our clients a far broader range of services.
"We look forward to being able to further strengthen our relationships with existing clients and winning new clients by expanding our strength in depth and providing additional specialist services."
There have been several mergers involving Scottish firms this year.
The tie-up between Burness and Paull & Williamsons was the largest this year between two Scots firms, creating a practice with £37m turnover, 60 partners, 158 lawyers and offices in Edinburgh, Glasgow and Aberdeen.
Dundee firm Blackadders and Edinburgh's McKay Norwell have also joined forces, while Tods Murray partnered with private client specialist Fyfe Ireland in one of the first Scottish mergers to take place in 2012.
Major cross-border deals include the McGrigors merger with Pinsent Masons and DWF merging with Biggart Baillie.