Graduating there as an M.D., he served as Surgeon with the 16th Battalion, Volunteer Militia during the Fenian Raids in 1866. He was also the first editor/publisher of the Picton New Nation as well as a teacher and school inspector. From 1878 to 1892 he was the Liberal member for Prince Edward County in the federal parliament.

Platt became warden in mid-1899. He was an advocate for rehabilitation. He felt strongly that work and discipline should benefit, not punish, the incarcerated, even though the job that employed the most convicts was breaking rock by hand on the stone pile. The renovation of the cells was completed in 1906, and he banned tours of the penitentiary by the public.

At Cedarhedge, Platt had two blood-hounds as pets which “the prisoners thought would devour them on slight provocation”! Evidence of the dogs can be seen on the South-west window sill in the gallery on the museum’s second floor.

A sensational escape in May 1912 led to Platt’s resignation and calls from the public for an investigation about alleged issues like partisan hiring of guards and collusion between some inmates and staff. The subsequent Royal Commission recommended sweeping reforms, but praised Platt.

His only child, Garfield, lived at Cedarhedge with him. He later served as the Kingston Penitentiary Surgeon from 1931-1942. J. M. Platt died Sept. 27, 1919 at Picton, Ontario.