The church began as a simple hall church and was later extended eastward with a straight-ended chancel of the same width as the nave, transforming it into what is known as a sal-church. This eastward extension may have taken place as late as around 1600, during a major rebuilding that also added the present sacristy on the north side and a porch to the south. The tower, with its distinctive timber lantern and copper-clad spire topped by a cross, was built in 1801–1803; at that time the old porch was demolished and its stones reused as building material, while the south entrance was blocked up. The church bells, previously hung in a separate wooden bell frame, were moved into the new tower.
The exterior walls are roughcast with smooth-rendered corners, painted in ochre and white using lime-based paint, and the roofs are covered with tarred wooden shingles. In keeping with medieval tradition, the north wall of the church has no windows.
By the mid-19th century the building had fallen into disrepair, and in 1860 it was taken out of regular use when nearby Vänge church was enlarged to serve the Läby congregation instead. Left empty for decades, Läby kyrka was eventually restored under the architect Birger Jonzon and reconsecrated in 1928 by Archbishop Nathan Söderblom. A description from 1912 recorded the church shortly before this restoration as crumbling and abandoned, home to bats rather than worshippers — a vivid glimpse of its neglected state at the time.
Several notable furnishings survive. The combined altar-pulpit was carved by Magnus Granlund in 1753 and stands on the medieval altar; its central relief depicts the Transfiguration of Christ. Sold to the Nordic Museum in 1909, the pulpit was returned in time for the 1928 reconsecration. Two medieval wooden sculptures associated with the church — a crucifix figure showing Christ crowned as king, and a depiction of St Anne with the Virgin and Child, both dating from the 1300s or earlier — are now held at the Swedish History Museum rather than in the church itself. A communion vessel from the early 1300s, later enlarged in 1749, and a cracked bell cast in 1574 for Hjälsta church, are also among the church's inventory.
The present organ was built in 1956 by Olof Hammarberg of Gothenburg, replacing an earlier small positive organ recorded in 1869; its case, designed the same year, stands on a raised chancel step at the south side of the church.