Most forces had to cancel officers' rest days and extend the length of shifts as the United States president's four-day trip prompted tens of thousands of protesters to take to the streets.
"The full cost of the operation is still being worked out but an early estimate is almost £18 million".
The senior officer added: 'These demands affected local policing to differing degrees across the country with the host forces most impacted.
'All forces reported high demand at the time of the visit from the requirements of the operation as well increased demand and calls from the public linked to the World Cup and hot weather as well as numerous events and festivals.
"The majority of forces had to cancel officers" rest days and extend the length of their shifts.
Still, Ms Thornton commended the officers and staff involved whose hard work "made sure that the public were kept safe throughout".
Trump largely avoided big cities due to protests and visited Prime Minister Theresa May at Chequers, met the Queen at Windsor Castle for tea and attended a dinner at the ancestral home of Winston Churchill, Blenheim Palace in Oxfordshire.
West Midlands Chief Constable Dave Thompson said officers are not as fast at responding to emergencies and fewer offenders are being brought to justice after a 19% real terms reduction in police budgets. This would further reduce the sustainability of forces and Ms Thornton said the NPCC is discussions are being held with the government on the matter.