Princess Beatrice, 24, wore a black and cream short-sleeved dress and completed her look with a maroon hat, a matching clutch bag and black heels.
Beatrice's sister Eugenie, 23, sported a patterned, off-the-shoulder dress and accessorised her summery look with a cream hat and shoes, much like the taupe LK Bennet shoes favoured by Kate Middleton.
Eugenie and Beatice's father, Prince Andrew, joined the royal group at the Surrey racecourse.
Comedian Jack Whitehall and his girlfriend, former Fresh Meat actress Gemma Chan, also attended as did TV presenter Laura Whitmore and Les Misérables star, Samantha Barks.
The Queen's outing at Epsom comes a day after her poignant visit to Woolwich barracks on Friday morning. During the London engagement, the Queen paid tribute to murdered soldier Drummer Lee Rigby, who was tragically murdered last week.
The monarch met officers and soldiers associated with 25-year-old Drummer Rigby, who was stationed at the South London barracks, at the end of her visit.
Beatrice and Eugenie joined the Queen at the second of her annual summer garden parties at Buckingham Palace on Thursday afternoon.
The two princesses were impeccably turned out for the exclusive yet informal event. Beatrice, wore a fashionable monochrome print dress, cropped jacket while her younger sibling was the epitome of daytime chic in a white dress, nude jacket and an unusual hat inspired by the lily flower.