Most of the land belonged to the Portland Estate and its successor, the Howard de Walden Estate and around 1716, a street called Chandos Street was begun. Nearby was a street called Harley Street. In 1726, it was decided to swap the names – so Harley Street became Chandos Street and Chandos Street was called Harley Street.

Along its length, there was an existing inn called the Half Way House opposite a track which led to Marylebone village.

In 1719, it was joined by a new inn called the Blue Posts which was situated where 35 Harley Street now is, on the corner of Queen Anne Street. Harley Street could not continue south for a few years since the Blue Posts was in its way.

In the financial slump that followed the South Sea Bubble, growth was slow. A pair of small houses was built in 1723 next to the Blue Posts (later 31 and 33 Harley Strteet). A bath house came next on the site of 29 Harley Street, fed by the City of London’s conduit.

Harley Street was largely built up only after 1750. The two blocks between Weymouth Street and Marylebone Road was originally called Upper Harley Street until 1866 when the whole street was combined and renumbered. The Duke of Wellington was living here, at 11 Harley Street, on the future site of no.34) in the early 1800s. Harley Street now extends from Cavendish Square to Marylebone Road.

In exchange for other land, The Crown gave its land on the west side of Upper Harley Street to the Portland Estate, but kept land on the east side to build Nash’s Park Crescent. The street north of Devonshire Street was at first to be called Ulster Street and to cross over the New Road into the park, but in the event the Harley Street name prevailed.

By 1830 there were 145 houses in Harley Street and one pub – The Turk’s Head.

In 1840, out of 103 houses in Harley Street, 37 had East India Company connections and 13 had slave-owning links with the West Indies. There were twenty MPs living in Harley Street at this date.

Harley Street then became the centre of private medical care in London. It was next to Cavendish Square, the heart of the upper-class section of Marylebone – the further south on Harley Street and thus nearer to the square, the better the medical address. There were about ten doctors in 1840 but almost 300 by the time of the First World War. When the National Health Service was established in 1948, there were around 1500. By 2018, there were more than 3000 people employed in the clinics, medical and paramedical practices of the the Harley Street area.