Minimizing decoherence due to coupling of a quantum system to its fluctuating environment is at the forefront of quantum information science and photonics research. Nature sets the ultimate limit, however, given by the strength of the systems coupling to the electromagnetic field. Here, we establish the ability to electronically control this coupling and $textit{enhance}$ the coherence time of a quantum dot excitonic state. Coherence control is demonstrated on the positively charged exciton transition (an electron Coulomb-bound with two holes) in quantum dots embedded in a photonic waveguide by manipulating the electron and hole wavefunctions through an applied lateral electric field. With increasing field up to 15 kV cm$^{-1}$, the coherence time increases by a factor of two from $sim1.4$ ns to $sim2.7$ ns. Numerical calculations reveal that longer coherence arises from the separation of charge carriers by up to $sim6$ nm, which leads to a $30%$ weaker transition dipole moment. The ability to electrostatically control the coherence time and transition dipole moment opens new avenues for quantum communication and novel coupling schemes between distant qubits.
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