The deal, first mooted earlier this year, will see all Steel Eldridge Stewart lawyers, property consultants and support staff remain in the Cupar office they occupy. Partners of the firm, which was founded in 2003, Hilary Eldridge and Jim Steel will move to become consultants for Thorntons.
Experienced Thorntons partner Margaret Allan will now be based in Cupar with the firm confirming it will offer a range of new financial and investment services.
Ms Eldridge said: "This merger has been a long time in the planning and we are delighted with the response from both our clients and staff. Cupar is an important location for Thorntons and it will very much be business as usual."
Jack Robertson, Thorntons' chairman, added: "Whilst Thorntons will become one of Scotland's largest full service firms, our eye is very much on delivering a local, yet specialist service.
"We greatly appreciate the support and encouragement we have received from clients following the merger announcements and we can assure each and every one that they will remain our priority."
In June Thorntons said it had plans to merge with Steel Eldridge Stewart as well as Fife-based Murray Donald, which has two offices in St Andrews along with sites in Anstruther and Cupar.
Thorntons expects the Murray Donald agreement to complete on November 1.
Once that is sealed, the enlarged firm, which will trade under the Thorntons name, will have around 40 partners and 400 staff across locations in Angus, Tayside, Fife and Edinburgh.
Mr Robertson remains chairman while Craig Nicol and Scott Milne are joint managing partners.