Former proprietor Clinton Slingsby has taken the decision to retire but remains associated with the firm as a consultant.
An auctioneer for 42 years, Mr Slingsby started his career in 1968 and joined Thomas Mawer and Son in 1978, becoming sole principal in 1984. During his time at the Lincoln auction rooms, he has presided over around 700 auctions.
Day-to-day management of Thomas Mawer and Son, which was established in 1864, is being handed to senior valuer and auctioneer John Leatt, who joined the company last year after 19 years with Dreweatts, latterly in Nottingham.
Administration staff at Thomas Mawer and Son will continue in their various roles and future expansion plans may also create new job opportunities.
New owner Colin Young, the principal of Golding Young in Grantham since 1994, said the two Lincolnshire firms had worked together informally for many years and share professional resources as well as joint promotional initiatives.
