The proximity of Essex to London probably influenced the county in siding with the Parliamentarians when war was joined, as part of the Eastern Association. There were exceptions, mostly members of noble families, who fought for the King, but they were heavily outnumbered.
The most famous episode of the civil war in Essex is the siege of Colchester. On the 12th June 1648, the Essex royalists marched to Colchester, with Fairfax and the parliamentary army no more than a mile behind them. They managed to hold the town in the name of the King until the 27th August, when they were forced to surrender, and their leaders were executed in the Castle Park. You can still see holes from musket shot on some of the buildings around the town.