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We present experimental investigations of quantum thermalization in a precisely controlled, interacting, 171Yb+ spin chain, with up to 22 ions. We quench the trapped ion spins in a quantum many-body Hamiltonian with single-atom addressing techniques and measure the long-term dynamics with single-site resolution. With a long-range transverse Ising model spin Hamiltonian, we observe emergence of an exotic prethermal phase in the quench dynamics. This non-trivial prethermal phase arises from an inhomogeneous effective potential landscape, due to a combination of the long-range interactions and the open boundary condition.