Much of present day Hanover, including 320 10th Street, was once owned by early Bentinck Township settler Henry P. Adams and his wife. The first commercial buildings were often log cabins, later to be replaced by stronger brick structures. The current Leifso and Leifso Law Office building was constructed by H.H. Miller in 1900. Miller was a real estate and loan agent whose motto was "Always Prompt – Never Negligent." After the Knetchel Furniture Factory fire of December 20, 1900, many of the valuables rescued were stored in Miller's new office. An important figure in the early history of Hanover, Miller was part of Hanover's first village council in 1900 and later became the Liberal M.P. for South Grey from 1904 to 1911.
John Mills began to work for Miller in 1897. John A. Paterson joined the business in 1910 as a junior clerk. A few years following Miller's death in 1916, Paterson and Mills formed a partnership under the name Mills and Paterson Notary Publics. When Mills retired in 1937, Paterson bought the business and changed the name to John A. Paterson Notary Public.
Fire Insurance records from 1925 indicate that a Salvation Army Hall may have been located on the second floor of the office.
Unlike many of the current buildings on Main Street, 320 10th Street's façade has remained relatively unchanged, preserving the late Victorian era architecture. In January of 1982, the office narrowly escaped a fire that consumed part of Hanover's Main Street.
In 1995, Dean Leifso purchased this historic building from fellow lawyer Donald Neilson. The building became home to Leifso and Leifso Law Office on May 1, 2012 when Corwin Leifso joined the practice.