Casanova was not, by his own account or anyone else's, conventionally handsome. What he had was presence. He was tall for his era, dark-complexioned, powerfully built, and commanded a room. The face that launched a thousand seductions was strong-featured rather than fine: a prominent nose, dark penetrating eyes, a broad forehead, and a jaw that squared off a face built for confidence rather than delicacy.
We know this from two kinds of evidence. The first is Casanova himself. His memoirs, Histoire de ma vie, run to thousands of pages and include frank self-description alongside the more famous erotic adventures. He describes himself as tall, dark, and strong, with an olive complexion and features that were "not handsome but something better." He was aware that his appeal lay in energy, wit, and physical magnetism rather than beauty.
The second line of evidence is portraiture. Several paintings are attributed to this period, most importantly a portrait attributed to Alessandro Longhi (son of the more famous Pietro) and one by Francesco Narici. These show a man consistent with the written record: dark hair, strong nose, dark eyes, a face that is arresting without being refined. The attributions are debated in some cases, but the portraits align well enough with Casanova's self-description and with accounts from people who knew him.
By his later years, living in relative obscurity as a librarian at Count Waldstein's castle in Dux (Duchcov), Casanova had thickened considerably. The face of the old memoirist was heavier and jowled, a long way from the lean adventurer of Venice and Paris.