The oldest surviving section of the church is the chancel, built towards the end of the 1100s, possibly originally as a chapel entered from the west. A sacristy was added early in the 1200s, the nave followed around the middle of that century, and the tower was completed towards its end. A porch, since demolished, was also built at some point. Unusually for a Swedish parish church, Kattnäs has retained a well-preserved medieval form overall.
The tower's tall spire has its own story: a 17th-century spire was destroyed in an accident in 1856 and replaced by the current one in 1874.
Among the church's furnishings, the single bell hanging in the tower is dated to the 1200s and is thought to be among the oldest in the region. The plain medieval font is the church's oldest fitting, complemented by a second font of red limestone added in 1667. A triumphal crucifix from the 1400s hangs above the chancel arch, while the altarpiece dates from 1785 and the pulpit from 1705. The pulpit's canopy bears the Hebrew tetragrammaton, the four-letter name of God, read right to left.
The church's only organ was installed in 1875, a gift from a local parishioner, and remains in place today, a modest but historically interesting instrument.