Their successful collaboration for Glamorous Night (1935) ("Shine Through My Dreams", "Fold Your Wings") began a fifteen-year partnership that included six long-running hits.

During World War II in 1940, Hassall served in an anti-aircraft gun emplacement with editor John Guest, architect Denys Lasdun, and socialite Angus Menzies.

Hassall's contributions included: Upon Westminster Bridge, Daffodils, and Ode: Intimations of Immortality by William Wordsworth; and Death Be Not Proud by John Donne.

Hassall lived at Tonford Manor, a house with a mediaeval stone tower situated by the River Stour on the outskirts of Canterbury.

Shortly before his death in 1963, Hassall spoke about the first Stour Music Festival, saying: The shared experience … a communion between listener and performer … impossible in a great assembly … (which) means the restoration of a large body of music to the private salon or to the church where it originally belonged.