Holyhead came into its own in the early 19th century, when Thomas Telford built a new road to connect north Wales with London. The Admiralty Arch was built in 1824, marking the end of Telford's new road. The arch was designed by Thomas Harrison to commemorate the visit of King George IV in 1821, when he set sail from Holyhead on the royal yacht, on a state visit to Ireland. The Admiralty Pier was constructed in 1821, and was used by the City of Dublin Steam Packet Company. The so-called "Irish Mail Line" of the Chester & Holyhead Railway ran from Holyhead railway station to the pier.

In 1845, an Act of Parliament led to the construction of the new port, and the adjoining new railway station was opened in 1851.
In 1853, Queen Victoria arrived in the port of Holyhead with the royal party, including her consort Prince Albert.

The breakwater was completed in 1873, and was declared open by the same Prince of Wales, now an adult, who officially opened the New Harbour on 17 June 1880.
During the Second World War, the Royal Dutch Navy began using the port as a base.