Dustin Lance Black, who rose from obscurity to fame when he won an Oscar for his screenplay for “Milk,” about slain San Francisco city supervisor Harvey Milk, has written a heckuva memoir.

I use “heckuva” because it’s an impressive, readable story but also because that’s the kind of corny phrase that peppers Black’s account of a family that hung together despite dramatic differences and grievous hardship.

Although much of “Mama’s Boy” centers on Black’s indomitable mother, Rose, it frequently widens its scope from the personal to the political — often, alas, in the lingo of a campaign stump speech.

Black credits values he learned from his conservative, religious Southern mother as being consonant with those that allowed him to come out of the closet, write a prizewinning movie about an outspoken gay activist and fight for marriage equality in California and at the U.S. Supreme Court.

“If my mom and I could set foot on the bridges between us, then perhaps our neighbors and those closest to us could, too,” Lance writes. “Perhaps our diverging Americas wouldn’t be doomed to destroy each other the way our news shows and politicians would have us believe.”

Anyone writing a memoir at 44 is subject to the “too young” criticism, one Black deflects by devoting much of his book to Rose, who was born in poverty, the daughter of Louisiana sharecroppers. Found to have polio at 2, she spent her childhood until her teens in hospitals far from home. She underwent gruesome, painful surgeries. Doctors gave her no chance to walk or to have children.

Three husbands and three sons later, Rose, who walked with the aid of canes and leg braces and drove an adaptive car (like a maniac), proved everyone wrong. Dustin arrived during Rose’s second marriage, to a physically abusive man.

Starting when he was a boy, Black and his disabled mother had a special closeness. The family moved to San Antonio. Rose became a Mormon. Black acknowledges that the church helped Rose and her struggling family, but it posed a big conflict once he realized he was gay.

Black has a screenwriter’s knack for telling a rollicking tale, whether about a bike accident or a first date. Bits of suspense animate his narrative.

Black and his family moved to Los Angeles in 1987. He couldn’t afford USC film school, even though he was accepted there, so he went to a community college and was later accepted to UCLA’s film school. He eventually got a job writing for “Big Love,” the hit HBO series about Mormons in Utah.

Warner Bros. nixed Black’s pitch for a Harvey Milk biopic, but he plowed ahead on his own, eventually attracting Gus Van Sant and Sean Penn to the project.

When “Milk” was set to premiere in 2008, California voters passed Proposition 8, which negated a state law allowing same-sex marriage. Prop 8’s main sponsor? The Mormon church. Black used his Oscar acceptance speech to promise marriage equality in California and across the U.S., a pledge criticized by many more moderate activists. He joined the legal battle that continued for several years until the landmark Supreme Court rulings that recognized same-sex marriage nationwide.

“Mama’s Boy” is most engaging when Black vividly tells his life stories, including a surreal one about being invited by elders of the Church of Jesus Christ of Latter-day Saints to attend a Mormon Christmas Spectacular in Salt Lake City. His tendency to mansplain his anecdotes with bromides about overcoming differences and national healing is less successful.

Claude Peck is a former Star Tribune editor and a member of the National Book Critics Circle.

Mama's Boy

By: Dustin Lance Black.

Publisher: Alfred A. Knopf, 406 pages, $27.95.