The original Sarah Mildred Long Bridge was the third span to carry motor vehicle traffic between Maine and New Hampshire at Portsmouth, replacing a river crossing at its location dating from 1822.

It was renamed in 1987 to honor Sarah Mildred Long, a 50-year employee of the Maine-New Hampshire Interstate Bridge Authority who rose from a secretary at its creation in 1937 to executive director.[when?

The main span lifted on an as-needed basis for ocean-going commercial traffic, and on a varying set schedule for recreational craft unable to pass beneath the rail span, which was left in an open position by default to accommodate small boats and minimize interruption of auto traffic caused by raising the lift.

[7][8] On April 1, 2013, a large tanker struck the bridge, causing severe structural damage and leading to a temporary closure of vehicular traffic.

[10] On August 21, 2016, the bridge became stuck in the closed position due to a mechanical failure after a shift in one of its trunnions caused a sheave and thrust block in the south tower to jam.

On August 22, officials managed to partially raise the main span to allow shipping to pass under, which is given priority by federal law over road traffic.

[17] It had been weakened when a caisson used in the construction of the new bridge dragged its anchor cables, which pulled out several of the trestle's bents,[clarification needed][18] sending B&M engine No.

[citation needed] An award of $25 million was made by the U.S. Department of Transportation for the railroad portion of the work, reflecting its role in supporting the Portsmouth Naval Shipyard downriver.

Maine and New Hampshire's DOT officials maintain the bridge was "commissioned" in late January 2018 following several simulated lifting scenarios, allowing the states to take possession.