Davies Arnold Cooper (DAC) will drop the KSB Law brand as it aims to organically grow the firm following the completion of its takeover.
DAC stepped into the acquisition talks five months ago following the collapse of a potential merger deal between KSB Law and South East firm DMH Stallard.
The deal was completed at the end of 2007 with KSB staff moving into DAC's Bouverie Street office during the Christmas period.
The combined firm will take DAC's annual turnover to £40m placing it alongside Speechly Bircham at the number 65 spot in The Lawyers' top 200 from a turnover of £32m. The firm will be home to more than 400 staff, 220 of which are fee earners.
DAC senior partner Danny Gowan told The Lawyer the firms "stated objective" was to grow KSB Law's St Albans office.
"It will remain open for property and construction business," he said. "The acquisition will enable us to do real estate deals from St Albans which are probably more price sensitive in London."
Gowan said DAC's corporate portfolio, which was previously only a minor part of the firm, would also be given a boost by the acquisition.