Mountnessing is a village in the Borough of Brentwood in south Essex, England. It is situated to the north-east of Brentwood, south-west of Ingatestone. A large proportion of the houses are situated on the Roman Road (this was the A12 road until the village was bypassed in the 1970s) between Brentwood and Ingatestone. The village is approximately equidistant between the two closest railway stations of Shenfield and Ingatestone.
Features of Mountnessing are its windmill and the parish church of St Giles. An annual village fete in held in July. In the "Windmill" field there is also a village hall, cricket pitch, football pitches and tennis courts.
St.Giles church is located mid-way between Mountnessing and Billericay.
The parish (#15 on the map) was in the Billericay Rural District from 1894 until 1934 when it was transferred to Chelmsford Rural District, and since 1974, when boundaries were redefined, it has been located in the Brentwood District or Borough.