The body of a man was found in undergrowth next to the motorway, police said.
The M1 was closed in both directions near Watford, between junctions five and six, at about 15:00 GMT on Monday.
Officers said the death was being treated as “unexplained” and drivers faced severe delays throughout the evening rush hour.
The southbound section reopened at about 18:30, with the northbound stretch reopening just after 22:00.
One of three lanes northbound remained closed on Tuesday.
A Hertfordshire Police spokesperson said: “We were contacted earlier this afternoon after the body of a man was found in undergrowth at the side of the motorway between junctions five and six.
“Officers are currently working to establish the circumstances of the man’s death, which are currently unexplained, and will remain in the area this evening and tomorrow.”