She beat Dane County District Attorney Ismael Ozanne and state Rep. Jon Richards, of Milwaukee in Tuesday's primary.
Happ will face Republican Brad Schimel in the Nov. 4 general election. Schimel, the Waukesha County district attorney, is the only GOP candidate in the race. Incumbent Republican Attorney General J.B. Van Hollen isn't seeking a third term.
Happ's victory comes in her first statewide run for office. Richards had been in the state Assembly since 1999 and gave up his seat to run for attorney general.
Happ's only campaign ad so far featured her riding a Harley-Davidson motorcycle.