The east–west highway travels 4.52 miles (7.27 km) and connects SR 525 in Mukilteo to Interstate 5 (I-5) in southern Everett.

[16] The boulevard route had previously been surveyed in 1882 for a wagon road connecting Mukilteo to Lowell (now part of Everett) by the county government.

[25] Planning for a new freeway to carry SR 526 was approved in 1967 during construction of the Boeing factory, which was expected to generate more traffic congestion.

[33][34] Construction of the permanent four-lane, 3.5-mile (5.6 km) controlled-access highway section commenced shortly after Peter Kiewit Sons won the contract in May 1968.

[43][44] Additional upgrades included a high-occupancy vehicle lane on southbound I-5, along with a corresponding queue ramp on SR 526,[45] and the construction of sound walls along the freeway.

The realignment was intended to facilitate a new ferry dock in Mukilteo and to avoid the population center of the city.

However, after consulting with an advisory group composed of representatives from local city governments, transit agencies, and lobbying firms, WSDOT elected to make improvements to the section between Seaway Boulevard and the eastern terminus of the route.

Such improvements include the construction of a new eastbound lane, the widening of the Seaway Boulevard eastbound on-ramp to the route, the addition of a ramp meter for the direct ramp to I-5 north, and the modification of the signal lights at the eastern terminus intersection with SR 99 and SR 527.

[50][51][52] A different seismic retrofitting project involving the wrapping of steel "jackets" on the columns of bridges carrying SR 526 over Airport Road, Hardeson Road, and Casino Road, along with an on-ramp from Seaway Boulevard to eastbound SR 526, began in February 2023 and was completed that December;[53] during preparations for the project, WSDOT contractor crews found excessive excrement on the girders and supports of two of the overpasses from roosting pigeons, requiring cleanup by a hazmat crew and the installation of gel discs underneath the overpasses to deter all birds from them.