From Lynnwood, the highway serves Mukilteo and becomes the terminus of SR 526 before taking its ferry route to Clinton on Whidbey Island.

SR 525 begins at the Swamp Creek Interchange with I-5, also serving as the northern terminus of I-405, located in Lynnwood in southern Snohomish County.

The ferry was later taken over by the Puget Sound Navigation Company, later sold to the state government in 1951 to form the WSF.

[23] SSH 1D traveled 54.73 miles (88.08 km) on Whidbey and Fidalgo islands from the Clinton ferry dock to an intersection with the Anacortes branch of PSH 1 at Sharps Corner.

[30] WSF ferry routes were added to its respective state highways in 1994,[2] eliminating one of two gaps along the route of SR 525, the other being a concurrency with SR 99 that was replaced by a partial cloverleaf interchange in 2000 during the widening of the freeway segment in Lynnwood.

[2] The Mukilteo ferry terminal, originally completed in 1957, was replaced by a new facility that opened on December 29, 2020.

[32] The new terminal, which includes a waiting area, longer car queues, and a transit center, cost $187 million to construct.

[33] In April 2024, Mukilteo mayor Joe Marine floated the idea of renaming Mukilteo Speedway during a work session with the city council to better reflect the city's history; it was axed two months later after it received overwhelming opposition among participants in an online survey and a Facebook post soliciting feedback.

A proposal to extend Paine Field Boulevard through Japanese Gulch and create a bypass of the Mukilteo ferry terminal has been a part of the city's transportation plan since 2001, but has not been built.