The former Ebbsfleet United boss takes over at New Lodge following the departure of Jamie O'Hara at the beginning of December.
The Essex outfit sit 17th in the Vanarama National League South with 11 points secured from the opening 12 matches.
A club statement said: "On completion of a thorough selection process, involving circa 50 candidates, Billericay Town have pleasure in announcing that Kevin Watson has been appointed as our men’s first team manager with immediate effect.
"Kevin brings a wealth of playing, coaching and managerial experience to the club. He shares our one club vision and the board’s ambitions for Billericay Town FC. Kevin will be joined by Tristan Lewis as assistant manager.
"We’d like to take this opportunity to thank Danny Brown and Ash Goss for their excellent work in standing in at a critical time. They gave us the space to be able to work through the managerial selection diligently.
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