By 1765, Almack was operating his third St James's venture. In the summer of that year, he had opened Almack's Assembly Rooms in King Street, marking the occasion with a grand ball, attended by the Duke of Cumberland, younger son of George II. Within a year of the ball, Cumberland was dead. But Almack's Assembly Rooms gloriously outlived him. Very soon, their pre-eminence was unquestioned. Society hurried to dances there. Fashionable London fought for membership.

In pronounced contrast to Almack's gaming club, the Assembly Rooms not only admitted both the sexes, but were dominated by a clique of formidable aristocratic women, among them Lady Pembroke and Mrs. Fitzroy, who vetted prospective members in unashamedly vicious manner. A report dated 1765 recorded: "The Duchess of Bedford was first blackballed, but is now since admitted, the Duchesses of Grafton and of Marlborough are also chosen. Also Lady Holderness, Lady Rochford are blackballed as is Lord March."

The Assembly Rooms outlived Almack's club (though that was to be revived at the beginning of the 20th century, as is explained below), which suffered what proved to be a fatal blow when one of Almack's employees, William Brooks, established his own club in October 1778. It remains there, in St James's, to this day. But even as the club perished, the Assembly Rooms were assuming unrivalled status, regarded by many as "the seventh heaven of the fashionable world".

By 1814, Almack's committee comprised the Ladies Jersey (Sarah, Countess of 1786-1867), Castlereigh, Cowper (later Lady Emily Palmerston) and Sefton, the Princess Esterhazy and the Countess (later Princess) of Lieven. They had no qualms about exerting their authority, as Wellington discovered that year. It was his first time in England for five years.