Crystal field driven magnetoelectricity in the triangular quantum magnet <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:msub> <mml:mi>CeMgAl</mml:mi> <mml:mn>11</mml:mn> </mml:msub> <mml:msub> <mml:mi mathvariant="normal">O

We report dielectric and magnetoelectric studies of single-crystalline ${\mathrm{CeMgAl}}_{11}{\mathrm{O}}_{19}$, a Kramers triangular magnet embedded in a polarizable hexaaluminate lattice. In zero magnetic field, the permittivity ${\ensuremath{\varepsilon}}^{\ensuremath{'}}(T)$ follows the Barrett law of a quantum paraelectric down to $\ensuremath{\sim}25\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, below which a broad minimum develops near $3\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ without evidence of static ferroelectric or magnetic order. Application of magnetic fields up to $9\phantom{\rule{0.16em}{0ex}}\mathrm{T}$ shifts this minimum to higher temperatures and broadens it, evidencing a tunable magnetoelectric response. The magnetoelectric coupling was characterized using results from magnetization measurements. The anomaly temperature ${T}^{*}$, extracted from the local minimum of ${\ensuremath{\varepsilon}}^{\ensuremath{'}}(T)$, exhibits a linear dependence on the squared magnetization ${M}^{2}$, consistent with the biquadratic magnetoelectric coupling allowed in centrosymmetric systems. This magnetoelectric effect, mediated by spin-orbit-entangled Kramers doublets interacting with a frustrated antipolar liquid, establishes ${\mathrm{CeMgAl}}_{11}{\mathrm{O}}_{19}$ as a prototype for exploring quantum magnetoelectricity in frustrated systems.