Route 100 lies wholly within the town of East Haven.

It begins at US 1 in the eastern part of town and heads west through the town center along Main Street before turning north along North High Street.

It crosses over US 1 without a direct connection and intersects Interstate 95 (I-95) at exit 52 before continuing north to end at Route 80 in the Foxon section of town.

Traffic heading for southbound I-95 is directed to use Kimberly Avenue, which connects to US 1.

In 1943, it was extended south along Thompson Avenue (formers SR 558) to the entrance of Tweed-New Haven Airport.