Cooper's first trial ended with a hung jury on the murder counts.
The Dodge County jury deliberated about three hours before returning the verdict. Sentencing is set for July 24.
The first-degree intentional homicide convictions carry mandatory life prison terms. Judge Todd Ehlers will get to decide if or when Cooper is eligible for parole.
Bromfield accompanied Cooper to his sister's wedding at a Door County resort in 2012. After she rebuffed his romantic advances, he strangled her. Cooper also sexually assaulted her afterwards - something the first jury convicted Cooper of.