St Mary's is constructed in red sandstone and has a slate roof. It is orientated with the ritual east facing the northwest. The plan consists of a five-bay nave with a north porch but without aisles, large north and south transepts, a chancel with north and south chapels and sacristies. There is no tower. At the west end are diagonal buttresses, an entrance, and pointed windows containing Geometric tracery. The windows along the sides of the nave have two lights. In the south transept is a four-light window, and the north transept contains two two-light windows with a rose window above. In the chapel is a five-light window flanked by diagonal buttresses. The chapels are gabled with two-light windows. The south sacristy has one and two lights, with a rose window in the gable.

Inside the church, the high altar and reredos date from 1865, and were probably designed by E. W. Pugin. They were separated in 1948–50 by Weightman and Bullen, who placed the reredos against the east wall. The stained glass in the east window dates from 1878, and is a typical design by the Belgian stained glass painter Jean-Baptiste Capronnier. The two-manual pipe organ was built by Franklin Lloyd in 1895, and is situated in a gallery on the north wall of the church at the west end.

The main window was donated by the Jump family. At the apex is a representation of the last supper. St Benedict and Scholastica are pictured below and included because of the Parish being founded by Benedictines.

The saints below are (left to right) St Baldwin (a Benedictine reformer), St James, St Mary, St Henry and St Anne. These last four Saints are name saints of members of the Jump family buried outside near to the church porch. The inscription reads "Donante Jacob Jump luceo deo in domo" which translates as Dontate by James Jump to bring light to God’s house.

Above the word Donante can be seen the makers inscription J. P. Carpronnier, Brussels. Jean-Baptiste Capronnier (1814-1891) was a Belgian stained glass painter. Born in Brussels in 1814, he had much to do with the modern revival of glass-painting, and first made his reputation by his study of the old methods of workmanship, and his clever restorations of old examples, and copies made for the Brussels archaeological museum. He carried out windows for various churches in Brussels, Bruges, Amsterdam and elsewhere, and his work was commissioned also for France, Italy and England. At the Paris Exhibition of 1855 he won the only medal given for glass painting. He died in Brussels in 1891.

The window which can be found above the Lady Altar depicts St John the evangelist, St Mary and St Joseph. An inscription read "Pray for the soul of Joanna Simpson who died September 8, 1859". A Miss Simpson gave a gift £1000 towards the building of this Church

The window above our Sacred Heart Altar has images of St Elizabeth, St Anne and St Robert. All three saints are saint names of members of the Roskell family. Also look at the base of the main altar for the inscription about Joanna Roskell

The window in the South Transept was erected to the memory of Francis and Clare Reynolds, buried outside by the church entrance, who lived in what is now Reynolds Park. The window depicts St Francis and St Clare. Francis and Clare Reynolds were members of the third order of St Francis. The other two saints Elizabeth of Hungary and St Vincent de Paul are patrons of the third order. In the small panes below each saint are scenes from their life.

Central to the rose window, found in the North Transept is Our Lady of Perpetual Succour. This icon was brought to Liverpool in 1866 by the Redemptorists.