An adjustable height bicycle seat post assembly comprises a hollow seat post for supporting a bicycle seat slidably supported on plastic shims inside a hollow tube. The hollow tube clamps into the frame of a bicycle. A main spring forces the post upward, but a locking mechanism interconnects the post with the tube in various fixed positions relative to the tube. The locking mechanism includes a foundation and a projection extending from the foundation having a counterbore for guiding a plunger into holes on the post. The locking mechanism adheres to the outside of the tube to bear shearing forces on the plunger. An endcap on the locking mechanism and a top cap on the tube protect the assembly from foreign debris. The locking mechanism may be manipulated remotely using a magnetic switch assembly or manually using a manual assembly.