Mountnessing is a village and civil parish in the Borough of Brentwood in south Essex, England.

A large proportion of the houses are situated on the Roman Road between Brentwood and Ingatestone; it was formerly the A12 until the village was bypassed in the 1970s.

The parish church of St Giles is located midway between Mountnessing and Billericay.

The village has three pubs (The George and Dragon, The Plough and the Prince of Wales), a butcher's and a hairdressers.

A short-lived greyhound racing track was opened during 1931, at Chain Bridge on the main London Road.