Holy Trinity Church, which dominates the High Street, was built by Bishop Gundulf c.1080. There is good evidence to suggest that a much earlier church occupied the site in Saxon times. The church provided a focus for the religious and ceremonial life of medieval Dartford. The building was significantly enlarged during the reign of Henry III (1216-72) to accommodate a new chapel dedicated to St Thomas of Canterbury for use by visiting pilgrims. Pilgrimages to Canterbury ceased in 1538 during the reign of Henry VIII. At this time, Becket’s altar was removed from the church, his festivals abolished, and the local trade in pilgrim souvenirs was halted. Consequently, many Dartford traders lost a valuable source of income.
Further extensions were added to the church during the reigns of Edward I (1272-1307) and Edward II (1307-27). In 1313, Thomas de Wouldham, Bishop of Rochester, visited the church to inspect a new window, which his chaplain Hamo de Hethe commissioned. About 1470, bells were hung in the newly heightened tower.
Regular maintenance and upkeep of the church property was an expensive business, the cost being borne by the parishioners. In 1470 the church roof had to be re-covered with lead. Shortly afterwards, the church tower was heightened. A document of 1453 confirms that the church administered its own cemetery sited right next to the church building.
Prior to the Reformation, the church had no seats or pews for worshippers. The congregation remained standing during services. At least four main altars and other shrines decorated the church. The high altar was dedicated to the Holy Trinity. Three additional altars were dedicated to St Thomas of Canterbury, St Mary, and St Ann. Statues of the Virgin Mary and St Anthony occupied a prominent position. A niche known as Sepulchre was used to display a crucifix during Holy Week. The front of the church was dominated by a large illuminated rood or cross.
Masses were said in the church for the souls of the departed. Singing or chanting in Latin formed an integral part of the worship. In c.1485, a magnificent fresco depicting St George slaying the dragon was painted on the east wall of St Mary's chapel. This painting can still be seen in the church today. Some of Dartford’s wealthy residents, like Thomas Bond, asked to be buried close to the main altar in the church. Prominent local worthies were commemorated in the church when they died. A fine commemorative brass can still be seen in the church today commemorating Richard Martyn who died 18 February 1402. A chantry or chapel known as Stampit Chantry was founded by Thomas de Dertford at Holy Trinity church in 1338.