Julian Oliver Mazzariello was born on October 31, 1978 in Hatfield, Hertfordshire, to an English mother, Anne, and an Italian father, Fernando, a drummer from Cava de' Tirreni near Salerno who had moved to London in the late 1960s. His parents bought him a piano when he was six and he began lessons the next year. His Universal Music Italia biography dates his decision to become a jazz musician to a Miles Davis concert he saw at eight, at London's Royal Festival Hall. At 13, in 1991, he won the Daily Telegraph's Young Jazz Musician of the Year prize with a friend, the drummer Jose Hathaway, according to Italian Wikipedia. A biography on the Musincantus site adds classical piano grades from the Associated Board of the Royal Schools of Music.

In 1995, at 17, he moved to his father's home town of Cava de' Tirreni and was quickly absorbed by the Salerno jazz scene. Through the drummer Pietro Vitale, a friend of his father, he met the Deidda brothers, Jerry Popolo, Giovanni Amato and the tenor saxophonist Daniele Scannapieco. In 1996 the singer and producer Gegè Telesforo heard him in a Salerno bar and asked him into his group, and Telesforo later introduced him to the saxophonist Stefano Di Battista. His first Discogs credits date from 1998: the Pietro Vitale Jazz Combo's Então Está, with the vibraphonist Francesco Lo Cascio and the bassist Attilio Zanchi, and bassist Giovanni Tommaso's film soundtrack La Prima Volta, on which he plays piano and keyboards. In 2002 he was pianist and co-composer in the High Five Quintet's Jazz For More... for Via Veneto Jazz, alongside trumpeter Fabrizio Bosso and Scannapieco, and played piano, keyboards, organ and steel drums on Telesforo's We Couldn't Be Happier.

The next years took him through Italian pop and television. After a summer 2002 concert at Rome's Testaccio Village, the singer-songwriter Lucio Dalla invited him onto his 2003 album Lucio and its tour, and he recorded piano for Alex Britti's 3 the same year. In 2005 he accompanied the singer Nicky Nicolai at the Sanremo Festival with "Che Mistero è l'Amore," with the Stefano Di Battista Jazz Quartet, and the song won the Groups category and finished fourth overall, according to Italian Wikipedia. He was also the pianist in Di Battista's quartet for the Canale 5 show Il senso della vita in 2005 and 2006. Alongside that work he stayed on jazz dates, among them Woman's Land (2011) with Di Battista, on which Discogs credits him on tracks 2, 5 and 7 to 11, and Scannapieco's Lifetime (2008), recorded in Paris with Di Battista, trumpeter Flavio Boltro, bassist Dario Rosciglione and drummer André Ceccarelli.

In 2011 he became the pianist of bassist Enzo Pietropaoli's Yatra Quartet, with trumpeter Fulvio Sigurtà and drummer Alessandro Paternesi. The group made Yatra (2011), Yatra Vol. 2 (2013), Yatra Vol. 3 (2015) and, with singer Cristina Renzetti, Yatra Songs (2022), and Top Jazz voted it best Italian group of 2013. Mazzariello, Pietropaoli and Paternesi also recorded as a trio billed under their own names, The Princess (2017, Via Veneto Jazz), a set of Lennon, Dylan, Cole Porter and Beach Boys songs. With the vocalist Maria Pia De Vito the same rhythm section made Dreamers (2020), which reworks songs by Bob Dylan, Joni Mitchell, Tom Waits and others, and Dreamers Live (2021), taped at the Pomigliano Jazz Festival on September 30, 2020 and issued with Musica Jazz magazine. With Pietropaoli alone he made two duo records for the audiophile label Fonè, Easytude Live (2020, recorded live on March 26, 2019 at the Auditorium Museo Piaggio in Pontedera) and Likewise (2022).

The partnership that has run longest is with Bosso. The pair's duo album Tandem (2014, Verve/Universal) was followed by Tandem (Live At Umbria Jazz Winter) (2019), and Mazzariello is the pianist in the Fabrizio Bosso Quartet with bassist Jacopo Ferrazza and drummer Nicola Angelucci. Discogs credits him on that quartet's Duke (2015, an Ellington programme with the Paolo Silvestri Ensemble, with Claudio Corvini and Gianni Oddi in the band), State of the Art Live! (2017), Merry Christmas Baby (2017), WE4 (2020) and We Wonder (2022), a Stevie Wonder programme, and Spotify lists him on Routes (2026). The duo also made Il Cielo è pieno di stelle (2024), a Pino Daniele tribute for Warner, and with the singer Fabio Concato the trio Non Smetto Di Ascoltarti (2016).

His own records are few. Debut (2018, Via Veneto Jazz and Jando Music) is a trio date with André Ceccarelli on drums and Rémi Vignolo on double bass, nine tracks of which seven are his originals; the blogger Steven A. Cerra heard a light touch in it and a debt to Cedar Walton. Reflections, recorded solo in September 2019 in Pontedera and issued on Fonè Jazz as an LP in 2021 and a hybrid SACD in 2022, pairs standards ("Body & Soul," "Autumn Leaves," "Theme From M.A.S.H.") with originals written between 1996 and 2019. In 2022 he made Avvenne a Napoli, a CD-and-book set of twenty Neapolitan songs, with the singer Eduardo De Crescenzo. Discogs also credits him on the trombonist Dino Piana's Al Gir Dal Bughi (2021), his last studio album, with Franco Piana, Enrico Rava, bassist Gabriele Evangelista and drummer Roberto Gatto. Italian Wikipedia and his biographies also place him in Rava's Standards project, in the group Lifestories with the guitarist Jim Mullen and in Ceccarelli's fusion band Troc, whose Crosstalk (2015) he plays on; none of the first two is documented at Discogs credit level. The label's release page credits him on piano on the drummer Marcello Di Leonardo's The Magic Box (2023), and Musincantus lists him on bassist Dario Deidda's debut 3 From the Ghetto (2003), though Discogs itemizes no personnel for that record.

Mazzariello is a pianist of the mainstream and post-bop tradition who moves easily between acoustic swing and electric-piano funk. His own record Debut mixes groove numbers ("Funky Chunks") with ballads and blues ("Monk in Paris," "Modal Blues," "J's Ballad"); the solo Reflections is a standards-and-originals program; and the Bosso quartet's We Wonder is a Stevie Wonder set on which Discogs credits him at piano and electric piano and tags the music Soul-Jazz and Jazz-Funk. He also plays organ (Giovanni Amato and Max Ionata's Organic) and keyboards, and Discogs credits him on a number of Italian pop records, among them Lucio Dalla's Lucio and Alex Britti's 3, and on two Neapolitan hip-hop albums, all of them sideman work outside jazz.

Italian and English sources name Miles Davis as the spark of his early listening; a blogger's review of Debut lists Hank Jones, Tommy Flanagan, Kenny Barron, Bill Evans and McCoy Tyner as reference points, which is that reviewer's reading rather than the pianist's own list.