The name Bangor traces to a phrase meaning place of points or horned curve, probably describing the shoreline of Bangor Bay. The Old Irish tale Táin Bó Fraích offers a more elaborate origin - the Connacht warrior Fráech and the Ulster warrior Conall Cernach returning from the Alps with Fráech's cattle, the herd shedding their horns as they came ashore on what is now Bangor Bay, giving the place its name Trácht mBennchoir, the strand of the horn-casting. Like many Irish placename legends it is a story told to fit a name rather than the other way around. The bay itself is what mattered. Sheltered from northern weather, opening east toward the Irish Sea, it became a landing place repeatedly through Bangor's history - for Vikings, for William of Orange's Marshal Schomberg, for Eisenhower's reviewing eye.

In 558, by the most likely dating, Saint Comgall founded the monastery of Bangor on the south shore of Belfast Lough. The Annals of Ulster put the date at 555. Whichever, the abbey grew into one of the three leading houses of Celtic Christianity, alongside Iona and Bangor on the Dee in Wales. Three thousand monks gathered to Comgall's rule, by the medieval chroniclers' reckoning. The choral psalmody developed at Bangor was carried to the Continent by missionaries who founded monasteries from Luxeuil in France to Bobbio in Italy. Bangor's influence was great enough that the place was named in the Hereford Mappa Mundi of about 1300 - one of only four places in all Ireland marked on that medieval map of the world. The Antiphonary of Bangor, the abbey's surviving manuscript of hymns and prayers, sat in Bobbio for a thousand years before scholars rediscovered it.

The modern town had its origins in the early seventeenth century, when the Lowland Scot James Hamilton arrived in 1605 having been granted lands in North Down by James VI and I. Bangor became a borough in 1612, electing two MPs to the Irish Parliament. The Old Custom House, completed by Hamilton in 1637 after the town gained port status in 1620, still stands as a memorial to that Scottish plantation. During the Williamite War in 1689, Marshal Schomberg's expedition landed at Ballyholme Bay with around 10,000 troops and captured Bangor before besieging Carrickfergus. By the 1780s, Colonel Robert Ward had improved the harbour and brought cotton mills to the seafront - steam-powered factories employing hundreds. The cotton trade peaked, then faded; by the mid-nineteenth century, the railway to Belfast in 1865 was bringing a different kind of population - day trippers and seaside holidaymakers, the working class enjoying inexpensive train fares for the first time in Irish history.

During World War II, General Dwight D. Eisenhower stood on Bangor's North Pier and addressed Allied troops who were about to depart for the D-Day landings. Ballyholme Bay served as an American training base. The young men he spoke to were heading for Normandy, and many would not return. In 2005, Eisenhower's granddaughter Mary-Jean Eisenhower came back to Bangor to oversee the renaming of the marina's North Pier as the Eisenhower Pier - a tribute to the speech and the soldiers and the place that held them. The pier still juts into the bay where the cotton mills once stood, where Schomberg's troops had landed, where Vikings had drawn up their boats centuries earlier.

Bangor declined as a tourist resort from the 1960s onwards as cheap foreign holidays drew the crowds elsewhere. The town reinvented itself as a Belfast commuter community - population around 14,000 in 1930, 40,000 by 1971, more than 64,000 at the 2021 census. The seafront has been repeatedly redeveloped, sometimes contentiously; in November 2009, UTV viewers voted the derelict marina frontage Ulster's Biggest Eyesore. The Flagship Centre closed in 2019. Springhill Shopping Centre, Northern Ireland's first purpose-built shopping centre, was demolished to make way for a Tesco. The Troubles brought eight deaths to Bangor, including that of Mildred Harrison, the first RUC woman murdered on duty, killed by a UVF bomb in 1975. On 2 December 2022, as part of Queen Elizabeth II's Platinum Jubilee Civic Honours, Bangor was granted city status by Letters Patent. The oldest place name on the Belfast Lough coast became Northern Ireland's newest city - a thirteen-hundred-year-old monastic foundation finally formally a city in its own right.