The oldest surviving portion of the church is the chancel and a small chapel behind the altarpiece, built in Romanesque style in the 1100s on the site of an old pagan place of sacrifice known as Torsharg. When Torshälla gained town rights in 1317, rapid growth made an extension necessary: a new nave was added to the west and the earlier nave became the chancel. During the 15th century the tower and porch were added, and vaulted ceilings were installed.
In 1614 the tower was rebuilt with a tall spire. Accounts of its height vary considerably, with some sources citing figures as great as 102 metres including the spire, while an 18th-century survey by builder Fredric Golling suggested a more modest 78 metres. Whichever figure is correct, the church was for a long time among the tallest buildings in Sweden and the highest church in the diocese of Strängnäs, its tower serving as a landmark for shipping on Lake Mälaren. A lightning-triggered fire in 1873 destroyed the spire and roof; the rebuilding that followed replaced the spire with the stepped brick gables seen today, and the nave roof was lowered. Traces of the earlier, taller roofline can still be seen on the tower's east facade.
The chancel vault carries painted ceiling decoration attributed to the workshop of Albertus Pictor, dating from the mid-1400s. Among the scenes is a depiction of Abraham reading with a pair of spectacles, said to be the oldest known Swedish image of eyeglasses in use, and a distinctive detail for visitors to look out for.
Four medieval wooden sculptures survive in the church, representing Saint Birgitta, Saint George, Saint Catherine and Saint Gertrude, giving a sense of the devotional imagery once common before the Reformation. The pulpit, dating from the 1600s, is carved with a central figure of Christ, the four evangelists and the apostle Paul.
The church also contains the graves of notable local figures, including Reinhold Rademacher, credited with founding Eskilstuna's metalworking industry, and Adolf Zethelius, a silversmith and ironworks owner. The current organ, built in 1980, reuses the case and some sounding front pipes from an earlier 18th-century instrument, linking the church's musical life across several centuries.