The Parish of Killowen is one of the smallest in the geographical area in the Diocese of Derry, yet on of the largest in population. Its church which is dedicated to St.John, is situated on the west bank of the River Bann in Coleraine, Co. Derry. The diocesan boundary with Connor Diocese is formed by the river, and the other side of which is the parish of St. Patrick's, Coleraine.
The name Killowen means, "Owen (or Eugene's) Church". The dedication to St.John, which has lasted, is a mis-translation of the Irish. In ancient times, the parish was called Drumtarsi. In 1248, the English erected a castle and a bridge over the Bann. Stones for this castle came from the ancient abbey of St. Carbreus.
At the time of the Plantation, the Clothworkers' Company was established in the area, which then was a county, the County of Coleraine. The name Killowen was given to the parish at this time. In 1622, Killowen Church was, "meanly repayred". In the 1768 survey, it was reported to be in very good repair. The church was converted into a school house in 1830, and in that year, a new church was built with financial assistance from the Clothworkers' Company, the Bishop, the Irish Society and the Board of First Fruits. In 1875, the church was practically rebuilt. The west wall was added, the east wall was partially replaced, and a chancel was added. The side walls were raised, and a new roof was built. The church was consecrated on 27th July 1875.
The vestry room to the left of the chancel, was built in 1927 in memory of Henry Stewart O'Hara, Bishop of Cashel, at the expense of his widow.