Military leaders hope that the fact the offensive has been widely publicised will persuade Taliban fighters to lay down their arms before the fighting begins.
"We very much hope that the military phase of this operation will go ahead swiftly and with as little incident as possible," Sedwell told reporters in a briefing at Nato headquarters in Kabul.
"The success of the operation will not be in the military phase.
"It will be over the next weeks and months as the people ... feel the benefits of better governance, of economic opportunities and of operating under the legitimate authorities of Afghanistan," he said.
Gulab Mangal, the regional governor, said a commission has been formed to handle the flow of displaced and any other fallout from the military action.
"The commission is fully prepared. We have got tents. We've got food. We've got everything in place," he said at the joint news conference with Sedwill on Tuesday.
"So far we have had two waves of displaced people from the area - 72 and 92 families," Mangal said.
Up to 100,000 people are believed to live in the area.
Authorities have not advised people to leave the Marjah area but have warned them to stay inside and avoid road travel once the operation begins.
"The message to the people of the area is of course keep your heads down, stay inside when the operation is going ahead," Sedwell said.
The military operation is the first phase of a plan to push out the anti-government fighters and return the area to civilian governance, Mangal said.
"The main purpose is to have Afghan sovereignty in the area and expand the sovereignty of the republic of Afghanistan," he said.
"We have to clear the area of the opposition enemies in order to be able to stabilise the area."