The church took shape gradually: the earliest section, encountered first on entering, dates from the 1100s, while the nave was extended to its present size in the 1300s, and a sacristy and tower were added in the 1400s. The tower and exterior have been altered several times since, most recently in 1889, when the tower gained the brick crown with recessed blind arcading, echoing medieval patterns, that it still shows.
The church's history has been marked by repeated disaster. A 1595 report already mentions fire damage, prompting Duke Karl (later Karl IX) to promise help with rebuilding, and lightning brought the bells crashing through the tower around 1680. The most catastrophic blow came in 1719, when Russian raiding forces burned the church during the Great Northern War, destroying the roof, furnishings and all older parish records. Rebuilding relied on local generosity, national collections and various gifts.
Because of this turbulent past, surviving older objects carry particular interest. A late fifteenth-century altar cabinet, made in Lübeck, came to Västerljung as a gift from Ösmo parish after the 1719 fire. The medieval vaults, now with their plaster removed, still shape the interior, and three brass chandeliers hang in the nave, the foremost thought to date from the 1500s. The present pulpit, from the 1830s, replaced an earlier one that had come from Trosa town church.
The two bells were cast by the workshop of "gelbmästare Gerhard Meyers enkia," the larger in 1721 bearing a Latin inscription referring to the Russian devastation, the smaller in 1768; after the 1719 fire the bells were kept safely in a separate tower before being rehung in 1768. A thorough restoration in 1954 introduced new pews and altar rail, rebuilt the organ loft and installed a new organ, whose facade nonetheless reuses parts from an 1884 instrument.
Outside, a runestone carved around the eleventh century stands south of the church; it was found built into the tower wall and set up nearby during the 1954 restoration, its inscription commemorating a father named Germar/Gerrnar. The Arctic explorer Nils Adolf Erik Nordenskiöld and his descendants are buried in the churchyard to the north, while just outside the churchyard boundary stands the Hallwyl family's grave monument, marking the resting place of Count Walter von Hallwyl and his wife Wilhelmina.