We present experimental data and numerical modeling of a nonlinear phenomenon in active magnetic microbead rheology that appears to be common to entangled polymer solutions (EPS). Dynamic experiments in a modest range of magnetic forces show (1) a short-lived high viscosity plateau, followed by (2) a bead acceleration phase with a sharp drop in apparent viscosity, and (3) a terminal steady state that we show resides on the shear-thinning slope of the steady-state flow curve from cone and plate data. This latter feature implies a new protocol to access the nonlinear steadystate flow curve for biological EPS available only in microliter-scale volumes. We use the momentclosure form of the Rolie-Poly kinetic model for EPS hydrodynamics, together with a decoupling approximation that obviates the need for a full three-dimensional (3D) flow solver, to qualitatively reproduce this dynamic experimental sequence. We thereby explain the phenomenon in terms of entangled polymer physics, and show how the nonlinear event (acceleration and termination on the shear-thinning response curve) is tunable by the interplay between molecular-scale mechanisms (relaxation via reptation and chain retraction) and magnetic force controls. The experimental conditions mimic movement of cilia tips, bacteria, and sperm in mucus barriers, implying a physiological relevance of the phenomenon and compelling further quantitative kinetic-flow 3D numerical modeling. (C) 2013 The Society of Rheology.