​​As a conductor, Marcello has conducted both standard repertoire and numerous premieres of new music written by fellow composers. He has worked with symphony orchestras including the Oxford Philharmonic Orchestra, Budapest Symphony Orchestra MÁV and Orchestra of the Vienna Opera Festival, instrumental chamber ensembles (e.g. formed of players of the London Symphony Orchestra), chamber choirs and conducted operas. Marcello conducted both the UK and Danish premieres of Amir Tafreshipour's The Red Room at the Tète-â-Tète Opera Festival and Aalborg Opera Festival, respectively. At the University of Oxford, he was conductor of the Oxford University Philharmonia and founded the Oxford Young Soloists Ensemble. With the latter, he received financial support from the university to produce, curate and conduct the world premiere of a previously lost and important work by Elisabeth Lutyens, alongside Stravinsky's Dumbarton Oaks Concerto and Bach's Brandenburg Concerto No. 3 in Europe's oldest purpose-built concert hall, the Holywell Music Room (Oxford). 

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Marcello was assistant to Toby Purser and Alessandro Pagliazzi at the Vienna Opera Festival, on Mozart's Don Giovanni and Humperdinck's Hansel and Gretel, leading numerous rehearsals and vocal coaching sessions.

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Marcello has conducted in venues including the Musikverein (Vienna), Borrömaus Saal (Vienna), Glasshouse International Centre for Music (Gateshead), Sheldonian Theatre (Oxford), Oxford Town Hall (Oxford), Cockpit Theatre (London), Durham Cathedral, and The Temple Church (London).