The deal with Berrymans Lace Mawer is scheduled to formally complete on May 1 and result in a new firm, to be called BLM, with annual revenue in the region of £100 million.
All 80 HBM staff are being retained with David Taylor, chair of the firm's management board, suggesting the agreement will unlock growth potential which could lead to additional hires in Scotland.
There was unanimous support for the merger from all 16 partners at the firm.
Mr Taylor said: "This is a real growth opportunity for us, that is why we are doing it. The combined firm will be able to support more clients and truly offer a cross-border service."
One of the factors understood to have driven the deal is that the firms work for several of the same large insurance clients.
HBM, which had turnover of £7.3m in its most recent financial year, is understood to be on panels for the likes of Aviva, AXA, Allianz and QBE.
Large insurers have increasingly been looking to buy legal services from one UK-wide firm or only admit practices with full UK coverage to tender panels.
That trend was among the factors behind Biggart Baillie merging with DWF and Andersons combininig with DAC Beachcroft in recent years.
HBM was at one point rumoured to be a target of DWF as it looked to add further capacity in Scotland.
Mr Taylor believes the merger will allow the firm to give more options to clients and said: "We might get a case in Scotland which might have an English law aspect. At present, unless we were to get English lawyers in, then we would not be able to provide that advice.
"With this combination that is clearly a huge advantage to us as we can provide that one-stop shop."
HBM's existing premises in Edinburgh and Glasgow will still be used although a serviced office in London, used by HBM lawyers while in the city on work, is to close shortly.
Berrymans has eight sites across England, including two in London, as well as operations in Cardiff and Dublin.
BLM will have 170 partners and a total workforce of around 1550. Mr Taylor outlined that an executive board and partnership board will be formed to lead the merged firm.
He declined to give specifics on the make-up of those but said there would be "appropriate" representation from the Scottish part of the business within each.
Mr Taylor said along with insurance there were growth opportunities in other areas HBM operates in such as the public sector, healthcare and family law. Siobhan Kelly, head of family law at HBM, said: "This provides us with an exciting platform of increased resources with which to service the needs of our clients in an increasingly complex and expanding area of law."
HBM Sayers was formed in 1999 after the merger of Hamilton Burns & Moore and Cochran Sayers & Cook.
Berrymans and Lace Mawer combined in 1997
Mike Brown, senior partner at Berrymans Lace Mawer, said: "We're excited about helping our clients to save time and money reducing risk, resolving disputes and managing claims.
"As BLM we will be able to do that across more lines of business and in more locations for more clients.
"The combination will give us a competitive edge at a time when Scotland's legal market is changing, presenting our business with significant opportunities."
McGrigors, which joined with Pinsent Masons, is the largest Scottish firm to merge with an English one so far. Dundas & Wilson is merging with CMS Cameron McKenna in May.