Isaias had earlier made landfall as a hurricane in North Carolina, where it caused floods and fires that displaced dozens of people.
Two people died when Isaias created a tornado that struck a North Carolina mobile home park.
Another died in Pennsylvania when their vehicle was overtaken by water and swept downstream.
Two others were killed by falling trees toppled by the storm in Maryland and New York City, and a sixth person died in Delaware when a tree branch fell on them, authorities said.
Isaias sustained top winds of up to 65 miles per hour more than 18 hours after crossing the coast, but it was down to 45mph winds late on Tuesday night, according to the National Hurricane Centre.
The storm’s centre was some 45 miles south-east of Montreal, moving northeast into Canada.
As Isaias sped northward, flooding threats followed. The Schuylkill River in Philadelphia was projected to crest early on Wednesday at 15.4 feet (4.7 meters), its highest level in more than 150 years.
Aerial video by WRAL-TV showed fields of debris where rescue workers in brightly coloured shirts picked through splintered boards and other wreckage of the Windsor, North Carolina, mobile home park where two people were killed.
Emergency responders searching the area found no other casualties, and several people initially feared missing had all been accounted for, said Ron Wesson, chairman of the Bertie County Board of Commissioners.
“It doesn’t look real; it looks like something on TV. Nothing is there,” Bertie County Sheriff John Holley told reporters, saying 10 mobile homes had been destroyed.
“All my officers are down there at this time. Pretty much the entire trailer park is gone.”
