Anton Mensing was a dealer, collector and auctioneer. In 1885 he joined the firm of Frederik Muller & Co, Amsterdam. He became a partner in 1892 with Adama van Scheltema, who had taken over the firm after Frederik Muller's death in 1881. After Schltema's death in 1899, Mensing effectively lead the firm, although it was not until 1905 that the first of several contracts between him and the Muller heirs the allowed him use of the Muller name was finalized.
Frederik Muller & Co. was begun as a rare books business, and it was only with Mensing that the firm expanded to include the auction of fine art, although this development was greeted with disfavor by some of the Muller family. In 1901 Mensing hired art historian Fritz Lugt [1884-1970], who was with the firm until the First World War.
During the years 1916-1922 Anton Mensing was deeply involved in the establishment of the Netherlands Maritime Museum. His own collection of scientific instruments was acquired en bloc by the Adler Planetarium in Chicago in 1929.
In 1929 Mensing's son Ben [Bernhard Franciscus Maria Mensing, 1903-1972] became a partner in the business. Mensing died in 1936 and Ben succeeded him at Frederik Muller & Co., which remained in business until c. 1960. The records of the firm were destroyed after World War II, although a private dossier of the Muller family was acquired by the Print Room of the Rijksmuseum (Inventory no. RP-D-1990:44) when it acquired Frederik Muller's print collection at his death.