This street was named for Mary Booth Hamblet (1840–1905), wife of Eli (1820–1905) and mother of Alonzo (1863–1937), namesake of Alonzo Avenue NW. The Hamblets were early Ballard settlers, and according to an article in the November 19, 1937 issue of The Seattle Times had their homestead where Ballard High School is today.
Mary Avenue NW begins at NW 67th Street just north of the high school and goes nearly 2 miles to the NW 105th Street right-of-way, where it becomes a trail leading to Carkeek Park. As I wrote in NW Blue Ridge Drive,
Ten or so years ago I saw a Private Property sign at the trailhead, put up by the Blue Ridge Club, but it was gone the next time I visited — I can’t remember if I complained or someone else did. (The woods are private, but the trail is Mary Avenue NW right-of-way.) This wasn’t in the original plat, but according to an old Flickr chat I had with Andreas “Severinus” Breuer, “there was apparently a WPA project approved to install a 30′-wide gravel road between 100th and 110th (apparently now NW Carkeek Park Road).… I imagine the ravine would look quite different if a 30′ gravel road had been put in, so presumably this plan wasn’t carried out. But a 1940 engineering map shows a surveyed ROW from 105th to the Carkeek border, and in Carkeek there seems to be a route that follows the WPA route (Clay Pit Trail > Hillside Trail > Brick Road Trail > Road). Perhaps the trail that exists today was made by the original surveyors or by WPA men?”