She was at a military parade in Richmond, North Yorkshire, to mark the amalgamation of the Queen's Royal Lancers and the 9th and the 12th Royal (Prince of Wales's).
The new regiment will be called The Royal Lancers.
Soldiers marched through the town before entering Richmond Castle for the takeover by the new commanding officer.
The lancers have historical links to the East Midlands, with the 9th and 12th Regiment mainly recruited from Derbyshire and Leicestershire and the Queen's Royal Lancers from Nottinghamshire and Lincolnshire.
On her visit to North Yorkshire, the Queen was wrapped against the spring chill in a soft wool coat and silk-wool dress by designer Karl Ludwig.
Her hat was designed by Angela Kelly and she wore a brooch of the Queen's Royal Lancers.
The royal engagement came just hours after Kensington Palace announced that the Duchess of Cambridge had given birth to a girl, weighing 8lbs 3oz (3.7kg).
Both Catherine and her daughter were "doing well", the statement added.
The Queen wore pink for a girl as she carried out official duties 250 miles away from her new great-grandchild.