A quote from the 2017 Fall Bid fundraiser for DiscNW, aka “The Northwest Ultimate Association” —

“The first unofficial Seattle ultimate “league” started in 1984 at Magnuson Park with an estimated five teams and fifty people. The demand for ultimate grew and DiscNW now has year-round leagues that serve over 8000 people.”

This is consistent with the slightly more elaborate history of Disc Northwest on Wikipedia (as of Nov 2017):

“DiscNW was founded by Joey Gray, Tom George, Mary Lowry, Jordan Dey, Maria Langlais, Mark Friedland, Bill Penrose and Lisa Thomas in 1995 as a nonprofit repository for funds generated by the growing Potlatch tournament. Soon, DiscNW became an umbrella for the spring ultimate league founded by Mark Friedland and others in 1984 and the fall league founded by Mike King. The first independent Juniors ultimate league was started in 1993 by Mary Lowry, Joe Bisignano, Jeff Jorgenson and others. In 2014, the DiscNW Middle School Spring League had over 1000 players on 79 mixed teams.”