The Romans arrived in AD 60 and built a fort here on Watling Street, where it crossed the River Can. The civilian town that grew around it was given an unusually grand name: Caesaromagus, Caesar's field or Caesar's marketplace. No other town in Britain bore the imperial prefix. The reason has been lost, but one theory holds that Emperor Claudius defeated the last organised British resistance on this site before marching on Camulodunum (modern Colchester) in AD 43. Beneath modern Moulsham lie the remains of the mansio - a Roman post office and traveller's hotel - and beneath the Odeon roundabout sit the ruins of an octagonal temple. In 2025 archaeologists working a former car park uncovered three Roman bodies. The town has never quite stopped being itself.

Around 1100, Maurice, Bishop of London, ordered a bridge built over the River Can. Traffic that had flowed through nearby Writtle now flowed here, and Chelmsford grew. King John issued a market charter in 1199, and by 1218 the town was the seat of the Essex assizes - the travelling royal courts. For six dark days in July 1381, after he had broken the Peasants' Revolt in London, King Richard II made Chelmsford the seat of his government, signing orders that ranged across the kingdom. Many of the revolt's ringleaders were hanged at Primrose Hill. In the 1640s, Matthew Hopkins, the self-styled Witchfinder General, used Chelmsford's gaol and assize court to send dozens of women - and a few men - to the gallows on charges of witchcraft, sometimes after barbaric forms of interrogation.

In 1899, the young Italian inventor Guglielmo Marconi opened his Wireless Telegraph and Signal Company in a converted silk works in Hall Street. It was the first dedicated radio factory in the world, employing about fifty people. By 1912 he had outgrown it, building the purpose-designed New Street Works - 70,000 square feet of clean industrial brick, its iconic water tower visible across the rooftops. Melba's broadcast came in 1920. Two years later, from a wooden shed at Writtle on the edge of town, Marconi's engineers began the world's first regular entertainment broadcasts under the call sign 2MT. Its sister station 2LO in London grew into the BBC. The signs at the city limits still read "The Birthplace of Radio." The claim is contested - other places have rival claims - but on certain technical points, Chelmsford is undeniable.

Chelmsford's factories made ball bearings, radar valves and radio equipment for the war. The Luftwaffe came on 14 May 1943, killing over fifty people and leaving nearly a thousand homeless. Worse was still to come. On Tuesday 19 December 1944, just before bedtime, the 367th V2 rocket to strike England fell on Henry Road, a residential street near the Hoffmann ball bearing factory and the Marconi works. There was no warning - V2s travelled faster than sound. Thirty-nine people died. A hundred and thirty-eight were injured, forty-seven seriously. Several houses simply vanished. Six days before Christmas, families learned that their children, parents or neighbours were gone. A memorial in the city cemetery on Writtle Road still marks the names. Hylands Park, on the western edge of town, had served as a prisoner-of-war camp and from 1944 as the headquarters of the Special Air Service.

In 1914, just as Europe stumbled into war, the parish church of St Mary the Virgin was elevated to become Chelmsford Cathedral on the creation of a new Essex diocese. A century later, on the Diamond Jubilee of Elizabeth II in June 2012, Chelmsford received its letters patent as a city. The journey is visible in the streetscape: the medieval flint of the cathedral tower, the Georgian Shire Hall facing the market place, the 1930s civic centre, the redeveloped Marconi site where only the listed water tower and the 1912 facade survive. Marconi's name finally vanished from the city in 2008 when its successors moved to Basildon. Teledyne e2v carries on in Waterhouse Lane, building thermal imagers and radar valves much as English Electric Valve did from 1947. Charles Dickens, visiting in 1835, called Chelmsford "the dullest and most stupid place on earth" - mostly because he could not find a Sunday newspaper. The slight has not been forgotten. But the town that hosted Caesar's marketplace, the first regular wireless broadcasts, and the SAS at its founding has had quite enough of being underestimated.