Spoiler warning: Plot and/or ending details follow.
Simmons emigrated to Tarsus IV from Earth at the same time as Adrian Kodos did, and supported Kodos when the latter moved to take over the colony in the face of a fungal infection which had destroyed much of the colony's food supplies.
At the last minute Simmons changed his mind about the necessity of killing 4,000 colonists. Kodos sent one of his other followers to eliminate Simmons, however Simmons was able to kill his pursuer and went into hiding with his girlfriend Joanna Robarge.
Simmons was located and arrested by a Starfleet team led by Lieutenant Commander Gabriel Lorca. Despite being assaulted by Lorca, who was dealing with his own grief and loss, Simmons agreed to help Lieutenant Commander Philippa Georgiou and Lorca track down Kodos. Simmons helped lead the team to the encampment where Kodos and his remaining followers had hidden themselves.
Following the apparent death of Kodos, Simmons was removed from Tarsus IV and sent to a neutral site for trial. He was found guilty and sentenced to a term of 20 years imprisonment for his actions leading up to the massacre on Tarsus IV. Due to his cooperating with Starfleet in hunting down former Governor Kodos, Simmons was sent to the penal settlement in north west New Zealand rather than a penal colony such as Elba II. A model inmate, Simmons never gave the staff any cause for complaint or concern.
Due to his excellent behavior, Simmons was considered for parole several times. Simmons personally appealed to the settlement director not to be paroled early, feeling that he should have to serve every minute of his 20 year sentence for his actions. Simmons acted as a mentor to new inmates, and helped provide security training for officers stationed at the settlement. (DSC novel: Drastic Measures)