The $156.7m purchase of 3.9m shares, unveiled last week, follows the acquisition of stakes by two other active investors, Nelson Peltz of Trian Partners and Mick McGuire of Marcato Capital.
Mr Loeb has indicated that he will enter into talks with Sotheby's board about the future of the company.
First-half results for 2013 showed a year-on-year fall of 7.7% in profits to $69.4m (£45.4m), with global sales standing at around 85% that of arch rivals Christie's.
As reported previously in ATG, Sotheby's released significantly more information on their website offering in their latest set of results than they have done in recent years, indicating that they have grown in confidence when it comes to their internet strategy. They have been notably trailing Christie's on this front in recent years.
Development of Far Eastern markets and private treaty sales, recent favourable restructuring of debt and the announcement that their headquarters in both New York and London could be sold if it made financial sense to do so all highlighted the potential for realising further value in the company's stock.