Leonard Dreyfuss was born in Brooklyn, New York, on November 6, 1886, the son of Henry and Fannie (Young) Dreyfuss. His father immigrated from Alsace-Lorraine in 1869. Leonard Dreyfuss was educated in the public school system in Brooklyn and at Pratt Institute in New York City.

Dreyfuss was employed by the John Matthews Co. of New York City from 1904-1912, serving as Vice-President from 1910-1912. The company was a manufacturer of soda fountains and related equipment for the manufacture and dispensing of soda water. From 1912-1914 he worked for the Charles E. Hires Co., manufacturers of root beer. He served as sales manager and New York manager for Hires.

Leonard Dreyfuss married Alice sometime between 1904 and 1919.

In 1914, Dreyfuss joined the Newark (N.J.) Sign Company, an outdoor advertising firm, as sales manager for the Newark district. The company grew rapidly and in 1917 Dreyfuss was instrumental in the merger with five other outdoor advertising firms to form United Advertising Corporation. From the merger to 1923, Dreyfuss was Vice-President of U.A.C. In 1923 he was elected President and Chairman of the Board, retaining the former until 1965 when he resigned. He remained Chairman until his death.

Dreyfuss was a man of tremendous activities and interests. He served as Director of New Jersey Governor's Civil Defense Advisory Committee (1956-1960). In the same vein, he was Chairman of the Newark chapter of American Red Cross as well as an honorary director for life. In 1942 Dreyfuss was named Newark's Outstanding Citizen of the Year. His interest in the city may be seen in his generous gifts to the Newark Museum, of which he was a trustee and treasurer. The Museum received a planetarium in 1952 from Mr. and Mrs Dreyfuss and a maintenance fund in 1960. The Museum's monthly concerts were supported by the Dreyfusses also. The French Government awarded him The Medaille de la Reconnaissance Francaise in 1949 for his work with the French Gratitude Train.

Dreyfuss was a benefactor to many colleges and universities in New Jersey. He was a trustee of Rutgers University, and a member of advisory boards at Seton Hall University and Caldwell College. Fairleigh Dickinson University made him a trustee (1966-1969) and Chairman of the Board of Fellows from 1961 to 1966. Dreyfuss received honorary LLDs from both Seton Hall (1950) and Fairleigh Dickinson (1954). Half a million dollars was contributed to the fund for the construction of the Leonard Dreyfuss College of Business at the Madison campus of Fairleigh Dickinson in 1967 by Dreyfuss. The monies for the construction of the Monsignor John L. McNulty Gateway at Seton Hall University was donated by Mr. and Mrs. Dreyfuss. Dreyfuss' interest in higher education went beyond material contributions. He was a frequent guest lecturer at many of institutions of higher education in the state.

Leonard Dreyfuss died on December 29, 1969 in Essex Fells, New Jersey.