He may have been the driving force behind the Mamas & the Papas—but when he wasn’t writing sunny flower-child anthems, John Phillips was intent on destroying himself and everyone in his orbit.

Michelle Phillips, 81, is the sole surviving original member of The Mamas & the Papas. Along with her, the iconic vocal group had John Phillips, Cass Elliot, and Denny Doherty as the original artists.

As the last remaining Mama and Papa, Phillips can get to work on the biopic she’s been trying to make for decades. “Now, mercifully, John is dead,” she says. “So he can’t stand in the ...