Dr. M.J. McKinnon resigned from active service as Surgeon of the 53rd Pennsylvania Volunteer Infantry on January 26, 1863 after suffering from exposure. On May 21, 1863, four months after his resignation, J.H. Taylor, Medical Inspector, Army of the Potomac, wrote to Dr. McKinnon, as printed in the Huntingdon Globe, August 23, 1865:
It affords me great pleasure to bear testimony of your efficiency as a medical officer. Our relations have been intimate for some time and my opportunities for observation extensive.
I can therfore say with truth that I always considered you one of the most reliable surgeons in the 2nd Army Corps. No better testimony to your kindness, zeal, ability, energy and efficiency is required than the grateful remembrance of the officers and men of the 53rd Regiment, Pennsylvania Volunteers.
I know the estimation in which you are held by the old regiment, and the knowledge that you comtemplated returning to the field would be held with pleasure by them as well as your numerous friends in the service.
“With much respect,
Your Obedient Servant,
Medical Inspector, Army of the Potomac