While at CCNY, he made his conducting debut for a production mounted by its Theater Workshop of Leonard Bernstein's first hit musical On The Town.

[3] Bernstein recommended Mandell to his mentor Serge Koussevitsky for a scholarship to the Berkshire Music Center in Tanglewood, Massachusetts.

His Ars Nova 1956 recording of Stravinsky's L'Histoire du Soldat, one of the earliest to employ stereo technology, has been re-released after 50 years.

In 1957, Bernstein appointed Mandell to become part of the creative team for his newly planned televised Young People's Concerts.

In 1973 Mandell became executive music director at the city of Leicester's newly opened Haymarket Theatre, which launched a number of major international revivals of musicals such as Joseph and The Amazing Technicolour Dreamcoat and Cameron Mackintosh's tour of Oliver!, prior to its London West End opening.

[note 7] After acquiring a major classical theatrical and light entertainment music library of over 1,000 orchestrations in 1976 from the estate of the British composer and arranger George Melachrino, Mandell launched a national family concert program conducting a reestablished Melachrino strings and orchestra ensamblee, with whom he toured the UK nationally annually until 2000.