Ted Olson, the conservative attorney who formed an unexpected alliance with a liberal counterpart to mount a legal challenge to overturn California’s ban on same-sex marriage, Proposition 8, has died. He was 84.

His law firm Gibson Dunn, where he was a partner, confirmed his death, but did not give a cause.

Theodore Boutrous Jr., another partner at the firm who also was part of the Prop 8 legal team, said in a statement, “Ted has been the heart and soul of Gibson Dunn for six decades and made us who we are today. He was not just an incomparable lawyer, mentor, role model, and friend, but he has made immeasurable contributions to the rule of law, our Constitution, and our country. We will miss him with all our hearts.”

Following the passage of Proposition 8 in 2008, a group of activists and entertainment industry figures, including Chad Griffin and Rob Reiner, teamed together...