[3][5] In Toronto in the fall of 1967, Mills-Cockell joined forces with light sculptor Michael Hayden, poet Blake Parker, and architect Dik Zander to form Intersystems, an arts collective and multimedia performance group.

In late 2015, all three Intersystems albums were remastered and reissued, together with a 132-page bound booklet containing photos, essays, and reproductions of vintage press coverage, on Italian label Alga Marghen, to widespread acclaim.

After a time, Canadian music executive and talent manager Bernie Finkelstein signed the trio to his newly created True North Records.

The album garnered an enthusiastic response from critics, and in its wake, the group found itself newly in demand from a variety of quarters.

Syrinx opened for jazz legend Miles Davis on his Bitches Brew tour, and played bills with Ravi Shankar.

[7] In addition, the first release attracted the attention of executives from CTV, which commissioned a theme song for its forward-looking television series Here Come the Seventies.

[7] The first album also resulted in a 1971 commission, from the respected Toronto Repertory Orchestra, of Stringspace, which Mills-Cockell composed for Syrinx, the TRO, and additional percussion.

[9] Undeterred, and with fundraising support from the Toronto musical community, the group purchased new instruments and pressed on to record the album.

In 2004, Mills-Cockell released on CD his seven-movement Concerto of Deliverance, an "extended tone poem with words," with libretto by Blake Parker.

He is currently working on bringing Savitri and Sam, a full-length opera he composed, with libretto by Ken Gass, to the stage.

Mills-Cockell has composed scores for such feature films as The Clown Murders (1976), Deadly Harvest (1977), Terror Train (1980), Humongous (1982), and Striker's Mountain (1985).

He has also been active as a composer for short film and television, and has created original scores for such productions as Packing Up, Reverse, The Italians, Stationary Ark, and The Little Vampire.

He describes first learning of the instrument: It turned out [Dr. Moog] was in upstate New York, which is not that far from Toronto, and we drove there in our Volkswagen psychedelic van to see him.

In 1966, his Fragments for Orchestra, Study for Bassoon, Prepared Piano and Magnetic Tape won a BMI Student Composers' Award.