A Royal Commission described it as a medieval nave which was enlarged with a clerestory by Prior Vyntoner the penultimate Prior of St Osyth.

The Church seems to have been built round about 1520-1530, by Vyntoner, though the lower part of the tower may have been earlier, (b) the "groined" roof has tierceron vaulting, where each main rib has a pair of minor ribs leading from the wall to the middle of the main rib. I believe it is unique in Essex. It also has, at the centre, a depiction of the Holy Trinity, with the Father on a throne, with the Son as a child standing before him and the Spirit as a dove hovering above him.
In front of the porch there are three niches. The central one has a heart underneath it, with a blade piercing the heart.

The empty niche probable held a statue of St Mary, who was told, in St Luke's Gospel, "This child will be for the falling and rising of many ...and a sword will piece your own soul also." The other two niches would have held statues of saints, but were removed by puritans in the 17th Century - Southminster was a Parliamentary town.

The 1819 extension was mainly in the east end, when the transepts and chancel were extended.

 The "modern" brickwork above the flint is Georgian, part of the enlargement of 1819.

The "Restoration" of the interior in 1892 was at the hands of William Lowder, what we could now call an Anglo-Catholic. He inherited a flat chancel from his predecessors, Alexander Scott and George Berkeley, in the style of most Anglican churches built after the Reformation and proceeded to 'Catholicise' it with a pair of steps into the Chancel, a further step into the Sanctuary and with a final step to the altar. He shortened the windows in the apse from those installed by Scott to allow a stone seat round the sanctuary and in memory of George Berkeley's widow allowed a reredos to replace an east window. He built a screen between the Nave and the Chancel and divided the transepts from the crossing. The South Transept became a chapel dedicated to St Mary, the "Lady Chapel" and the North Transept became a choir vestry.

The is no record of the date that the organ was installed, but it is the best part of 200 years old.

The most famous Rector was Dr Alexander Scott who was priest on HMS Victory at the time of Trafalgar and was responsible for the spiritual care of Nelson.

The local legend is that Reverend Scott is responsible for bequeathing to the church several items form Victory including a chest, a table, a glass fronted cupboard, a looking glass and a iron box fireplace.

Sadly the legend does not seem to be true as David Lumb, Vicar in the 1980s, enquired of the Victory about the table. It was never on that ship - far too big and all they had in 1805 is still there. Christie's told me that it was an office table, a partner table where pairs of workers would sit opposite each other to share work. Not mentioned is a cupboard in the Nave which contains a drawer with a top but no bottom. The top has holes for a glass bowl and two drinking classes. It is a "chest commode" such as was used in country houses where a maid would bring hot water to the bedroom in the morning and probably belonged to Scott's Vicarage, although the man from Christie's hazarded a guess that Scott had if from Emma, Lady Hamilton. They were certainly on very good terms and he was concerned for her daughter Horatia.