The unit at Grantham and District Hospital closed for the first time last night, in a trial which bosses said was needed due to a lack of doctors.
PCC Marc Jones said the force was not consulted and officers will waste hours taking suspects to the nearest A&E in Lincoln.
The trust promised to discuss concerns.
United Lincolnshire Hospital Trust (ULHT) will now only open the A&E at Grantham from 09:00 to 18:30 daily, for the next three months.
The trust says it does not have enough doctors to staff the department safely and also maintain services in Lincoln and Boston.
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Protesters claim the decision will put lives at risk, something which is denied by the trust.
It said the temporary closure at Grantham means it can maintain safe staffing levels at its two other A&E departments, both of which take a higher number of patients.
About 130 people gathered outside the hospital last night to protest the closure.
Mr Jones said: "To do this without consulting key partners such as police is very poor really, and we do need to work better together to protect the people we are all ultimately there to serve.
"When you say to the people of Grantham, you are going to have less policing as a result of the closure, then that is a significant impact."
Michelle Rhodes, the director of nursing at the trust said it was happy to discuss the concerns.
"We need to work with the police, look at the amount of patients that are transported here and work through that issue," she said.
"As a trust we will look  at it and take it seriously and work through it with the police. We have spoken to the police and it's not an issue that's been raised with us...but we will pick it up afterwards with him."
Meanwhile, Martin Hill, the leader of Lincolnshire County Council, has said that although he "reluctantly" accepts the decision, he remained concerned about the timings of the overnight closure.
He said: "I do not think it's right to have no medical access to anybody at all at Grantham at say 19:00. The 120,000 population needs something better than a locked door and being told to call 111."
The closure of a hospital's A&E department at night will waste hours of police time, Lincolnshire's Police Commissioner (PCC) has said.