Cousins has joined Bromley after their National League rivals continued their impressive summer recruitment.
Elliott Johnson, 24, will remain at the Hive as the academy product pledged his future to the club.
They will need a new goalkeeper after Cousins left for Bromley.
He told the Kent club’s website: “I’m really pleased to be joining Bromley, a Club that seems to be going in the right direction.
“I’m very much looking forward to working with Neil and of course Jim Stannard, who I worked with at Dagenham.”
Manager Neil Smith added: “Mark comes with experience, he’s played at this level for a while now.
“I needed to freshen things up at the back and get a little more experience in, and hopefully we can keep more clean sheets next season.”