William Cawthra (29 October 1801 – 26 October 1880) was a philanthropist, business and civic leader and the eldest son of Joseph Cawthra. William, like his father, was associated with reformists and was considered anti-establishment, notwithstanding his wealth. William was elected to Toronto City Council as the Alderman for St. Lawrence Ward in 1836, a position his father held for one year until he was unseated in 1835 by conservative opposition.
William married his widow Sarah Ellen Crowther Cawthra in 1849. His brothers John and Jonathan Cawthra served in the War of 1812 at Detroit and Queenston under Sir Isaac Brock. John Cawthra later served as an MHA and as the first Member of Parliament for Simcoe County.
William worked alongside his father Joseph in their family's business - Toronto's first apothecary. When Joseph died in 1842, he left most of his money to William who shut the business down and concentrated on investments and charity work.
William eventually became the wealthiest man in Toronto, serving as a Director of the Bank of Toronto and owned a number of companies. He and his wife lived in a townhouse at the corner of King and Bay Streets (the current site of the Bank of Nova Scotia's main headquarters). This house has been recognized as the most beautiful ever built in Canada, but was torn down in 1946 to make way for the Scotia Plaza. (Anthony Patrick Cawthra Adamson, a descendant of Joseph Cawthra, tried to rescue the mansion from demolition, but failed. He managed to save some architectural elements from the house and installed them in his Rosedale, Toronto garden).
William Cawthra is credited, alongside James Gooderham Worts and William Gooderham, Sr., with establishing Canada's first infectious disease centre at the Toronto General Hospital. William was a founding and active member of the Toronto House of Industry, established in 1837 with a similar model to Dicksonian workhouses, helping those in the city who were desperate for food, distributing coal to assist the needy to survive the harsh Toronto winters, providing temporary and permanent accommodations and assisting abandoned children or those who were orphans. William was also a patron of the Fenian Raids Volunteer Fund, St. James Cathedral and numerous other charities.