[6] In 1958 and 1960, Alleyn was included in the selection of Canadian paintings featured in the Guggenheim International competition,[7] and in 1959 he won the bronze medal at the São Paulo Biennale, then in 1960, with Graham Coughtry, Jean-Paul Lemieux, Frances Loring, and Albert Dumouchel, he represented Canada at the Venice Biennial.

[8] In the period 1962 to 1964, Alleyn’s growing interest in North American Indigenous art was reflected in ideographic and biomorphic forms and more colourful work, then by the mid-1960s he created "cybernetic" figurative painting.

On his return to Canada, he was struck by the changes caused by the Quebec’s Quiet Revolution and in the early 1970s, created proto-installations depicting realistic, colourful figures taken from photographs of crowds attending the Expo 67 site,[6] which he then painted on Plexiglas and installed in front of representations of sunsets.

At the Musée des Beaux-Arts in Sherbrooke in 2004, he showed his final series, "Les Éphmérides" (1995-2004)[10] which consisted of 12 large canvases as well as ink washes that echoed his abstract and figurative work of the 1960s.

[4][12] In 2016, the Musée d'art contemporain de Montréal organized a second retrospective titled Dans mon atelier, je suis plusieurs (In My Studio, I am Many), curated by Mark Lanctôt.