The Romans came first, building Derventio Carvetiorum at what is now the adjoining village of Papcastle to guard the river crossing on a major route toward Hadrian's Wall. The Normans displaced them, occupied the Roman fort, and then built Cockermouth Castle closer to the river. The castle's distinctive tilting tower - leaning Pisa-like over what would later become Jennings Brewery - still survives, though much of the rest was reduced thanks to the efforts of Robert the Bruce during the cross-border wars. From the medieval period the town grew its characteristic plan: a broad main street of burgesses' houses, each with a long burgage plot stretching back to a 'back lane' on either side. That medieval geometry is still legible. Walking Main Street today is walking a 13th-century layout, just dressed in Georgian and Victorian frontages.

In a tall Georgian house on the main street, William Wordsworth was born in 1770. His sister Dorothy followed in 1771. Wordsworth House, now in the care of the National Trust and restored after the 2009 floods, preserves a working 18th-century kitchen and a children's bedroom equipped with the toys and clothes of the family's time. The Wordsworths were not the only notable Cockermothians. Fletcher Christian, who led the mutiny on HMS Bounty in 1789, was born in the area in 1764. John Dalton, who introduced the atomic theory to chemistry, was born in 1766. Add Fearon Fallows, an English astronomer who became Astronomer Royal at the Cape Observatory, and the cricketer Ben Stokes, who grew up here after moving to Cockermouth as a child, and a town of fewer than nine thousand people has produced a curiously dense roll-call of consequence.

A market is recorded here before 1221, when Henry III granted charters that moved trading day from Saturday to Monday. The advantage of a Monday market endured into the 20th century: when pub opening hours were tightly restricted elsewhere, Cockermouth's pubs could open all day on market days, and the town became a popular destination for Bank Holiday drinkers. The Market Bell, inset into a wall opposite the Allerdale Hotel, remembers that era. In 1881 the town set up six electric street lamps - making it one of the first communities in Britain to experiment with electric lighting - but the service was unreliable and Cockermouth returned to gas. In 1964 the Council for British Archaeology named it one of fifty-one 'Gem Towns' in the UK, recognising the integrity of its historic core.

On 19 and 20 November 2009, a stationary band of rain settled over the western fells and refused to move. The Cocker and the Derwent both burst their banks. Water raced through the centre of Cockermouth at flood levels, and more than 200 people had to be rescued. RAF helicopters from Valley, Boulmer, and Leconfield lifted about fifty residents from rooftops; the rest came out by boat, including those of the RNLI. Many historic buildings on Main Street were gutted. Recovery was slow. By the summer of 2011 most were restored, but some remained boarded up for years. In December 2015 it happened again, when Storm Desmond pushed the Derwent over its banks and damaged hundreds more homes and businesses. In October 2023 the rear of the Old Courthouse collapsed into the River Cocker. The town keeps repairing, and the rivers keep coming.

What you see today is a centre that has worked hard at its own survival. Market Place has been pedestrianised and re-paved, with stonework that commemorates Dalton's atomic theory, local dialect, the floods, and a deliberately eclectic mix of town history. A statue of Lord Mayo - a former MP for Cockermouth who became Viceroy of India and was assassinated there in 1872 - stands on the tree-lined main street. The Kirkgate Centre hosts theatre, music, and cinema. Jennings Brewery, which spent nearly 150 years under the leaning castle tower, closed in 2022, though new owners announced plans in early 2025 to bring brewing back. A shared-use path runs along the trackbed of the old Cockermouth, Keswick and Penrith Railway - the line whose closure in 1966 was immortalised in Flanders and Swann's 'Slow Train' - and crosses a high bridge over the Cocker. From up there, you can see all of it: the rivers, the castle's tilting tower, and the houses where poets were born.