Bangura made sixty-two appearances for the Hornets during a four-year period.
He then had spells at Brighton & Hove Albion, Blackpool, Turkish side Mersin İdmanyurdu and Azerbaijani outfit Gabala respectively before joining Forest Green Rovers where he made eighty-three appearances.
He then signed for Coventry City in 2015.
He is also a Sierra Leone international.
St Albans manager Harry Wheeler said: “Al is another good addition to the group. He’s experienced, very professional and gives us something different. He also has a desire to go back in to the professional game and push on so we have no doubt he will give everything to be at his best for the cause whilst he’s with us.”