It is not known whether a church or chapel existed here in Anglo-Saxon, Anglo-Danish or Norman times. Although the Domesday Book of 1086 (reign of William I) lists two manors at "Sevinstone/Suivinestone" (Simpson) in Seckloe Hundred, it does not mention a church or priest here! The records of the Diocese of Lincoln mention, that in 1231, during the reign of Henry III, Philip de Lemington was presented as Rector of the church by Geoffrey de Cauz, Lord of the Manor.
The church was valued at £5 6s 8d in 1291 (reign of Edward I) and at £17 6s 8d in 1535 (Reformation of Henry VIII).
By 1847 Simpson, or Sympson, like other ecclesiastical parishes in Buckinghamshire, had been transferred to the Diocese of Oxford. St Thomas is one of the five churches in the Woughton Ecumenical Parish, which was established in 1977.
Grade 2 listed, St Thomas is of Oolitic Limestone construction and cruciform in plan - aligned east to west. It comprises a tall slender Central Tower; Nave, North and South Transepts and Porch with clay peg-tile roofs; and a Chancel with low-pitched Welsh slate roof.
The Central Tower, the oldest part of the existing building, appears to represent all that survives of a smaller cruciform plan church. The double-chamfered pointed arches resting on the semi-circular sectioned responds or jambs of the crossing, are in the late Early English style and date from c1250 - 1300. Four pointed windows of c1400 pierce the walls of the bell chamber. The Tower contains a peal of six bells - one bell of 1604, two bells of 1630 and one bell of 1694, all four recast in 1895/6; a new bell of 1895 and yet another of 1926.
The Nave is later than the Tower and is entered through north and south doorways which are typical of the Decorated style of 1300 - 1350. The single window piercing the north wall and two windows piercing the south wall, although repaired, show some portions of tracery of 1300 - 1350. However, at the west end of the Nave is a limestone font, with plain tapering round bowl, cylindrical stem and stepped base, typical of the 13th century. The elaborate wooden cover dates from the 17th century. Other 14th century features in the Nave include the cinquefoil arched niche for the holy water stoup by the south doorway and the doorways leading to the North and South Transepts (chapels). The large four light window at the west end of the Nave dates from the late-15th century and is in the Perpendicular style although extensively repaired externally. Also of the 15th century is the magnificent nave roof, with hammer-beam trusses at the east and west ends and three intermediate trusses of double arch-braced collar-with-tie-beam type. The 14th century roof was both higher and of steeper pitch!

A 15th century stair turret in the angle of the Nave north wall and the North Transept west wall gave access to a former rood (cross) loft, below which was a screen. It now gives access to the staircase of 1904 which leads to the bell ringing chamber in the Tower.
Above the Tower west crossing arch and unusually painted on the plaster are the Royal Arms of George II, dated 1742. The fourth quarter of the circular shield depicts the Hanoverian Royal Arms, in particular the white horse. The mural was carefully restored in 1953 (Coronation Year), when the outer GR2 was changed to ER2!
At the north-west corner of the Nave is a fine pipe organ, which was installed in 1920 by Messrs Bishop and Son, the organ builders, of Ipswich and London. The organ is said to have come from "the church at Frinton-on-Sea, Essex" and is the last surviving pipe organ in the three historic churches of the Woughton Parish.