Kelly McNeil has operated in Hastings since 1921.
The firm's current partners, husband and wife team Guy and Margaret Wellwood, said they welcomed the opportunity to join forces with lawyers they rated highly.
"The merger gives us every opportunity to quickly become the leading property and commercial law practice in Hawke's Bay," Mr Wellwood said.
Langley Twigg, incorporating Kelly McNeil, will start with a team of 32, including six partners and seven solicitors.
Mr McLauchlan said that, unlike mergers where a larger business was looking to rationalise, the aim was to give a broad reach and strong presence together with the advantages of being close to clients.