[4] The roof is of Welsh slate with coped gables, while the structure is built of flint and ironstone rubble walls with ashlar dressed stone and buttresses.
[3][5] The church's interior has a 19th-century timber barrel roof above both the nave and chancel, while the latter still has its original corbel masks and carved seats.
The nave is high and light owing to work done in 1866-70 when the window opposite the porch was added and the roof raised.
The free-standing communion table has a prominent position and behind it is a carving of an elephant and castle on the president's chair, which was made in 1974 using medieval woodwork already existing in the church; on a side panel is the head of John the Baptist on a platter.
Recent research suggests that these carvings were originally made in about 1475 for St Andrew's church in Biggleswade and were brought to Willian in about 1830.
The newer of the two lecterns was given in memory of Samuel Valentine, who died in 1974; the older one was given in 1870 by Thomas Legh Claughton, the Bishop of Rochester in whose diocese Willian then lay.
At the base of the tower the Ellacombe Chime bell-ringing apparatus on the wall enables one person to ring all six bells.