Isabella Stewart Gardner Museum director Anne Hawley, whose 25-year tenure began with a notorious art heist and culminated in a successful $180 million capital campaign, announced Wednesday that she plans to step down at the end of the year.

Hawley said she has been quietly weighing the decision for two years now, as the museum completed fund-raising efforts that included $114 million for the museum’s sleek 2012 expansion, designed by renowned architect Renzo Piano, and an additional $50 million to fortify the museum’s endowment.