During the 1950s, Hallberg played with leading visiting American players, including the tenor saxophonist Stan Getz, recording "Dear Old Stockholm" (originally "Ack Värmeland du sköna") with him, and alto player Lee Konitz in 1951, and trumpeters Clifford Brown and Quincy Jones in 1953.

[3][5] In the same period he worked with baritone saxophonist Lars Gullin, another leading Swedish player of the time.

Both players were associated with the 'Cool Jazz' scene in their country, influenced by the American school around pianist Lennie Tristano, a Hallberg favourite[6] with whom Konitz was associated.

Hallberg had a versatile style and in his later years he wrote music for film and television, as well as choral arrangements, and he also played the accordion.

According to Chris Mosey, while Hallberg was: "usually an extremely delicate and very measured player, [he] was obviously affected by the general ambience, and here and there cuts loose with awesome force".