The chancel remains from a large C13 cruciform church, whilst the north aisle and base of the tower are C14, the north chapel and some windows C15 and the south aisle and rest of the tower early C19.  There were later C19 and C20 alterations.

The church is close to the great house and its tower dominates the town.  Though a church is mentioned in Domesday Book (11, 18), no part survives from that period.  The chancel – with restored south lancets and two-light windows with circles in the head - and the north transept date from c1250, so the church was already cruciform.  The transept was divided horizontally from the start, for the upper part has a piscina (see below) and its restored upper windows are like those in the chancel. 

The nave was probably aisleless until the Percys (3 p17), who owned the house, added a C14 north aisle as a chapel.  It is clear from the west wall that it has been heightened and originally had a lean-to roof.  This is shown on one of the two Sharpe collection drawings, which also depicts both north and west porches, which have now gone (there is still a West doorway, but it appears entirely C19).  Inside, the form of the arcade is still C14, with octagonal piers (renewed) with abaci and upward extensions, from which the heads of three hollow-chamfered orders emerge.  The similar arch into the transept has no abaci.  The mid-C14 tower, of which the sandstone base remains, may replace a south transept, though towers in this position are found locally at Lurgashall and Tillington.  As built, it was not tall and until the C19 had a tall leaded spire, shown on the Burrell Collection drawing.

The renewed panelled head of the C15 east window is as shown on one of the Sharpe Collection drawings (dating from 1804 or 1805), though the rest was then blocked.  The other Sharpe drawing shows the outline of a large blocked west window, so the present C19 panelled tracery could well represent what was there before.  The east window of the gabled north chapel is similar in style, with the addition of a horizontal element in the head and there are three-light pointed north ones.  These are not old and the Burrell drawing shows only one, with a further square-headed one.  The arcade to the chancel has an octagonal pier and responds with rather crude abaci and the arch into the transept is similar.  The low east vestry is part of the chapel; a C17 monument with a scrolled top has been set in it.  A renewed south window of panelled tracery and an octagonal stair attached to the tower may be early C16, for there was a bequest of two shillings ‘fabrice campanilis’ in 1508 (SRS 43 p298).

Before 1800 there were few obvious further alterations beyond new windows, though the plain mullioned gable-window suggests the north transept may have been adapted as a pew in the C16 or C17.  In 1804 (certainly no earlier) the spire was replaced by battlements and pinnacles (2); according to Horsfield (II p179), the work was done in 1800, but Dr David Parsons's researches in the archives of Petworth house have established that the slightly later date is correct.  Thus, the two Sharpe drawings (one is missing and known only from a photograph), are probably the earliest depictions of the changed top.  There were further alterations to the body of the church in 1821, when T Chrippes inserted galleries (ICBS) - Quartermain ((W) p170) shows the dormers that lit them.   In 1827 Sir C Barry (A Barry p75) added a stuccoed stage to the tower, panelled on each side around a clock-face, and a tall broach spire of stone.  It has been suggested (see footnote in BE p294) that Barry's design for the spire was originally intended for his St Peter, Brighton, started in 1824, where a spire was never built.  However, although both designs show some similarities in that both are octagonal with lucarnes, the design for St Peter (the best reproduction of Barry's own print of the design is to be found in Dale (p32) and there is an even clearer view of the tower with spire from the west on the dust-jacket of his book) shows that the present parapet and pinnacles belonged to the original design; the spire was intended to rise from behind these and there were to have been low flying buttresses linking with the spire behind each pinnacle.  By contrast, Barry's design for Petworth had broaches that covered the whole top of the tower, with pinnacles at each corner of the broach.

The arch into the nave, hidden by the organ, may be his and so is the south aisle with C15-style windows.  It is taller than the north one, which he may have heightened.  Dr Parsons (2) also ascribes the present west window of the nave to him.  Little of his work remains inside, except, probably the chancel arch and the family pew as it now is, with a west stair and doorway.  The cost of this work, some £16,000 (KD 1899), which is a considerable amount even when paid by the Earl of Egremont, suggests that it was wider in scope than is now apparent.  Finally, during the earlier C19 at an unspecified date the sculptor J Carew (see under Monuments below), who was closely associated with the 3rd Earl of Egremont, is said to have altered the chancel (Roscoe p195), but there is nothing identifiable.   

There are three tantalising references to later C19 work.  First, the tower was restored in 1866 (B 24 (7 April 1866) p ii) and an undated restoration by Sir George G Scott is mentioned without further detail in his obituary in The Builder (36 p360), for which there is no known source (RIBA Library typescript p35) and which is likely to be one of the numerous misattributions to him.  Finally, in 1879 a faculty for unspecified repairs and restoration was granted (BN 36 p588).  Even if all these references are correct, it is unlikely that work at any of them was extensive, for in 1903 C E Kempe and W Tower (CDG 128 (1904) p98) renewed the roofs.  That of the nave has an arched ceiling with angels on the wallplates and the pargetting on the chancel, pretty but hardly a Sussex feature, must be theirs too.  Kempe, famed for his glass and fittings, designed both at Petworth (see below) and re-arranged the monuments.

The north transept gallery received a new front in 1903 and later a west one was built.  In 1935 C R B Godman and C J Kay rebuilt the top 15ft of the spire (CDG Nov 1935 p407), but in 1947 it was all taken down.  The present top of 1953 is by J Seely and P Paget (BE p294), who stripped the stucco from Barry’s upper stage, revealing an attractive pinkish brick, and added a plain parapet and low pyramid.  In 1958 the nave roof was decorated.

Fittings and monuments

Font: C15 octagonal with a quatrefoil on each side and an arcaded stem.
Glass:
1.  (North chapel) three C15 shields.
2.  (Round window over chancel arch) Mayer and Co, c1840, in lurid colours.
3.  (South chancel, second window) Clayton and Bell, 1872 (B 30 p1032).
4.  (South chancel, third window) Heaton, Butler and Bayne, 1874 (B 32 p975).
5.  (East window) Kempe, 1903.
6.  (Three roundels in north aisle) C E Kempe, c1885, brought here from his house, Old Place, Lindfield, in 1973 (Collins p312).
7.  (North transept, under gallery) Sir J N Comper, 1905-06 (BAL/MSS EeF/1/1).
8.  (South aisle, first window) Kempe and Co, 1912.
9.  (South aisle, second window) N Westlake, 1921 (WSRO Fac 2776).
Monuments:
1.  (North chapel) Sir John Dawtrey (d1542) and his wife and related to the contemporary series of monuments in the Selsey area.  Small figures kneel either side of a relief of the Trinity or Resurrection, now hacked away.  Cherubs holding shields are included in the otherwise gothic crested canopy and there was a helm above (see 1).  The painting is modern.
2.  Thomas Dawtrey (?1738) by P Hoare (Roscoe p623). 
3.  (Chancel) John Wickins (d1783) by J Flaxman (SAC 97 (1959) p84).  An angel holding a book floats on clouds, a motif the sculptor used elsewhere.
4.  (Chancel) Rear-Admiral Richard Willis (d1829) by J Carew (Roscoe p196).
5.  (Nave) Dr John Johnson (d1831) by J Carew (ibid). 
6.  Ann Child (d1835) by J Carew (ibid).
7.  (Tower) Percy family, 1837 by J Carew (ibid). Draped figure, holding a cross, which is said to have been intended originally for a monument in Wexford, Ireland.
8.  Charles Dunster (d1816) by Sir R Westmacott (ibid p1361).
9.  (North aisle) Seated effigy of the Third Earl of Egremont (d1837) by E Baily (ibid p58), best known for the statue on Nelson’s column.
10. (Nave) John Henry Robinson (d1871 and dated 1875) by C A Fellows (signed).  A bust and unusually late for this type.
Piscina: (North transept, upper level) C13.
Reredos:
1.  1903, gilded, by Kempe.
2.  (Tower) Small relief of the Virgin and Child by Flaxman.  From the reredos of 1827 and placed here in 1903.
Royal Arms: signed by Coade and Sealy, 1812 in their artificial stone (A Kelly p190).
Screen: Kempe, 1903.

Sources

1.  W Huyshe and Baron de Cosson: Notes on the Helms from Petworth Church, AJ 39 (1862) pp184-91
2.  D Parsons: St Mary's Petworth before Barry, Petworth Society Magazine
3.  R Turner: Petworth, SAC 14 (1862) pp1-24 
 

I am indebted to Dr David Parsons for making me aware of his article cited as no 2 above and for other interesting thoughts about the church.