Hildegard Schmahl was born on February 6, 1940, in Schlawe, Pomerania, which was then part of Germany. In 1945, the family fled from the Red Army to Hamburg. There, Schmahl trained as an actor at the Hamburg Acting Studio Frese from 1958 to 1960. After a first engagement at the Deutsches Schauspielhaus, she worked at stages in Braunschweig and Bern (Switzerland). In 1967, she performed Gretchen in "Faust I" at the Schauspielhaus Bochum; in the same year, she was awarded the North Rhine-Westphalia State Prize for the Stage. During the 1969/70 season, Schmahl played the title roles in "Emilia Galotti" and "Minna von Barnhelm" at the Staatliche Schauspielbühnen Berlin. In the years and decades that followed, she also frequently played female characters in classical plays at theaters in Berlin, Stuttgart and Hamburg, often under the direction of her husband Niels-Peter Rudolph.  

Parallel to her stage activities, Hildegard Schmahl also occasionally appeared in television and cinema productions from 1971 on, mostly in smaller roles, for example in the marriage and emancipation story "Familienglück" ("A Happy Family Life", 1975) and Harun Farocki's "Zwischen zwei Kriegen" ("Between Two Wars", 1978). With her move in 1987 to the Viennese theater Der Kreis with George Tabori, she took a break from film for several years. Under Tabori's direction, she played King Lear in his Shakespeare adaptation "Lears Schatten" in 1989. In the 1990/91 season, Schmahl transferred to Hamburg's Thalia Theater, where she worked with directors such as Jürgen Flimm, Leander Haußmann, Stephan Kimmig, Martin Kušej, Yoshi Oida and Tomaž Pandur.  

From 1992 on Schmahl also took on sporadic film and television roles again. She was part of the ensemble of Gábor Altorjay's "Punta Grande" (DE/HU/AR 1996) and Caroline Link's "Nirgendwo in Afrika" ("Nowhere in Africa", 2001), and played the actress Salka Viertel in Heinrich Breloer's "Die Manns – Ein Jahrhundertroman" ("The Manns – Novel of a Century", 2001, TV). She joined the ensemble of the Münchner Kammerspiele for the 2001/2002 season. In 2010, after Roma Bahn, Marianne Hoppe and Gisela Stein, she received the Hermine Körner Ring from the Berlin Academy of Arts, for her lifetime achievement.  

Among Schmahl's onscreen appearances in the noughties are the roles of the enigmatic grandmother of pharmacist Sternheim (August Zirner) in Ben Verbong's children's film "Herr Bello" (2007), a once-hidden ex-terrorist in Matti Geschonneck's "Entführt" (2009, TV), a mysterious matriarch in the "Tatort" episode "Das Wunder von Wolbeck" (2012, TV) and a grandmother in the family story "Der Geschmack von Apfelkernen" ("The Taste of Apple Seeds", 2013). She was part of the large ensemble of the improvised TV movie "Altersglühen - Speed Dating für Senioren" (2014) and Dominik Graf's thriller "Am Abend aller Tage" (2016).   

In the multiple award-winning social drama "Agnieszka" (DE/PL 2014) she played the owner of an escort service, and in Matti Geschonneck's East German family history "In Zeiten des abnehmenden Lichts" ("In Times of Fading Light", 2016) she played the wife of an imperious family patriarch (Bruno Ganz). She also had leading roles in Till Endemann's literary adaptation "Wir sind doch Schwestern" (2018, TV) and in Kai Wessel's miniseries "Die verlorene Tochter" ("The Lost Daughter", 2020, TV), in which she played a senior citizen and grandmother with a dark past.  

Despite her numerous appearances in front of the camera, Schmahl remained loyal to the theater. In addition to her roles at the Münchner Kammerspiele, she appeared on stage in 2016 as a messenger in "Mauerschau" at the Bavarian State Opera. In 2021, she appeared in Jossi Wieler's production of Hugo von Hofmannsthal's "Das Bergwerk zu Falun" at the Salzburg Festival.   

In 2022, she appeared in two feature film productions: In the controversial children's film "Der junge Häuptling Winnetou" ("Young Winnetou and the Lost Buffalo"), she played a shaman, and in Lars Jessen's film adaptation of the bestselling novel "Mittagsstunde," she had a leading role as the protagonist's (Charly Hübner) mother in need of care.