In 1968 - a time of political unrest in the USA - Castoro abandoned colour and started to engage with Concept Art, Street Works, Concrete Poetry and Post-Minimal sculpture. In 1969 and 1970, she participated in Lucy Lippard’s first number shows in Seattle and Vancouver, seminal exhibitions that helped define the terms of conceptual art. Her first solo exhibition was at Tibor de Nagy Gallery, New York in 1971, where she exhibited her imposing Free-Standing Walls and Giant Brushstrokes. Alongside gallery shows, Castoro always manifested an interest in making and showing art in the street, from her early performative Street Works to her later participation in the event Art on the Beach at Battersea Park, New York in 1978. Her totemic Flashers were displayed across New York City in various configurations and also in Paris on the banks of the Seine river in 1983.

Throughout her career Castoro lectured at the School of the Museum of Fine Arts, Boston; University of Colorado, Boulder; Syracuse University; Pratt Institute and Hunter College, New York.

She has participated in numerous solo and group exhibitions, including Distillations at Tibor de Nagy and Stable Gallery, New York (1966); Number 7 at Paula Cooper Gallery, New York (1969); Painting 1967-68 at Hal Bromm Gallery, New York (1975); Direction 1981 at the Hirschhorn Museum and Sculpture Garden, Washington, D.C. (1981); Side by Side: New Acquisitions, The Museum of Modern Art, New York (1985); Knee-High Flashers at Galerie Arnaud Lefebvre, Paris (1993); Site and Insight: An Assemblage of Artists, MoMA PS1, New York (2003); Configurations, MAMCO, Geneva (2005); Land of Lads, Land of Lashes, Galerie Thaddaeus Ropac, London (2018). Her first major museum retrospective was at MACBA, Barcelona in 2017, followed by a survey show at MAMCO, Geneva later this year.