Foundation: around 1100; claims for a pre-Norman foundation rest with the Westminster Charter, which dates itself to 1067, but is now seen as a 12thC forgery. The first named incumbent appears to be Robert de Albano, 1328. Panoramas of 1544 and 1616 sketchily show a crenellated profile with a massive tower, somewhat like that of St. Giles Cripplegate, much rebuilt but still standing in the Barbican today.

London Bridge: For some 700 years, St. Magnus the Martyr stood on the north end of London Bridge, which seems to have followed the same line from its Roman origins through a series of early wooden bridges to Peter Colechurch's stone bridge of 1176-1209. Until the building of Westminster Bridge in 1750, this was London's only bridge; sections periodically collapsed for want of maintenance, and its sides were lined with (revenue-generating) shops and houses as late as 1760. A new bridge was finally built 1823-31, by Sir John Rennie to his father's plan, in parallel just to the west (both to minimise disruption and to create a grand new road between the bridge and Bank). The old bridge was demolished in 1832, but two of its stones, rediscovered in 1921, now stand in the churchyard. Rennie's bridge was replaced in 1967-72 by that of Mott, Hay & Anderson, and now stands in Lake Havasu City, Arizona. The model of the old bridge in the lobby is by David T. Agget.

Fire and War: St. Magnus the Martyr was one of the first casualties of the Great Fire of 1666, which started in nearby Pudding Lane, and raced down Fish Street Hill to consume a third of London Bridge, as Pepys recorded in his diary entry of September 2nd: "it hath burned down St. Magnes Church and most part of Fishstreet already". Another fire destroyed the roof in 1760 (allegedly after an apprentice in an oil shop left a flame unguarded, in order to watch the procession of the unfortunate Lord Ferrars to Tower Hill, where he was executed for murder after a trial by actual Peers at Westminster); various small fires occasioned other repairs as recently as 1995. The church suffered bomb damage in both world wars, particularly in an air raid of 1940, but was restored by 1951.
Location: The church stands on land originally reclaimed from the Thames foreshore by the Romans. When Londinium began, soon after the conquest of 43 AD, the river actually ran along what is now Lower Thames Street, and a port was established where Pudding Lane stands today. Over two centuries, the Romans built quays and, later, a river wall to complement the 2ndC city defences; a blackened fragment of their revetments (excavated by Gerald Dunning at Regis House, 1929-31) now stands in a corner of the church porch. But the City declined with the Empire, and a shifting tidehead literally moved the Anglo-Saxons to establish their Lundenwic at the present Aldwych ('old town'). But King Alfred's expulsion of the Danes in 886 marked another turn in the City's fortunes, and the growing population resumed the building of quays and warehouses. By the time of the Norman conquest in 1066, the site of the church stood on solid ground, well above the new foreshore.