The church is one of the finest examples in Europe of late Norman architecture. Its soaring columns and rounded arches convey majesty, strength and simplicity. Building began in 1120 but through lack of funds the three bays at the west end were only completed during the reign of Henry III (c. 1250) by which time the style of the arches had changed from Norman to Early English. The king gave a large number of oaks from the New Forest to provide the roof timbers.

The stone for this new Abbey was brought from Binstead on the Isle of Wight. It was floated across the Solent on barges and travelled over land by ox cart. The nuns dedicated the church to St. Mary and Ethelflaeda, although the townspeople for some reason unknown, preferred to call it the Church of St. Lawrence. St Lawrence was a Deacon who suffered for the Christian faith in the early centuries in Rome.

The Norman solidity of style is relieved at the east end by elaborate decoration of arche arches and by lively carvings on the capitals. Two treasures preserved from the earlier Saxon church are the carved stone roods or crucifixes, one within the church and one outside.

In the later Middle Ages there were complaints that the Romsey nuns were frequenting local taverns. The reputation of nearly all religious houses was low, giving Henry VIII the excuse to close and pull them down. However, the people of Romsey raised £100 to purchase the church building and since then it has taken on new life as the parish church of the town.