“No gay person in this country would be married without Mary Bonauto,” said Roberta Kaplan, who went before the justices Wednesday to argue one of the cases.

As the top civil rights lawyer for Gay and Lesbian Advocates and Defenders, or GLAD, based in Boston, Bonauto has spent more than a decade plotting a careful strategy to advance gay marriage rights. She prompted Vermont to create civil unions in 2000, won the 2003 case that made Massachusetts the first state to legalize same-sex marriage and last year persuaded a federal appeals court that the Defense of Marriage Act, which denies federal benefits to gay couples, is unconstitutional.

Yet in a quirk of fate, Bonauto is watching her life’s work this week from the court’s spectator seats.

The justices considered a Defense of Marriage Act case Wednesday, but it was not Bonauto’s, which she argued when Justice Elena Kagan was President Obama’s solicitor general. Instead the court took up a similar case, Kaplan’s from New York, presumably so Kagan would not have to recuse herself.

“Am I disappointed?” Bonauto asked last week in the kitchen of her home in Portland, Maine. “There is an element of disappointment, but I’m also incredibly excited. I feel like after all these years, you’re getting a hearing — a fair hearing — from the highest court in the land.”

At 51, Bonauto is unassuming, serious and low key.

“She is not going to set a room on fire,” said Dean Hara, a plaintiff in Bonauto’s Defense of Marriage Act case. “But when she is arguing, she is really somebody to listen to.”

Even her opponents offer kind words, saying they appreciate her civil tone.

“She has always been the consummate professional, very courteous and gentle,” said Kris Mineau of the Massachusetts Family Institute, even as he said her courtroom victories had “degraded the value of marriage.”

Bonauto would be the first to say she builds on the work of others. As early as the 1970s, gay couples began suing for the right to marry, inspired by the 1967 Supreme Court ruling in Loving vs. Virginia, which struck down state laws banning interracial marriage. In 1983, a young Harvard law student named Evan Wolfson wrote his third-year thesis on why gays should be free to marry.

Wolfson was eventually hired by Lambda Legal, the gay advocacy group, where he joined a network of people dedicated to toppling laws that criminalized homosexual sex. Among them was Bonauto.

The daughter of a pharmacist and a preschool teacher from Newburgh, N.Y., she had come out, with some difficulty, while an undergraduate at Hamilton College. There, Bonauto was harassed over her sexual orientation — an experience, she said, that contributed to her desire to “make life better” for others. Although her office is in Boston, Bonauto works mostly from her home in Portland, Maine, where she and her wife, Prof. Jennifer Wriggins of the University of Maine’s law school, are raising their 11-year-old twin daughters.

Of all her legal victories, the 2003 Massachusetts case sticks with her. She still remembers clutching the Supreme Judicial Court’s ruling in her hand, and can still recite its compelling first paragraph, including the declaration that the Massachusetts Constitution “forbids the creation of second-class citizens.”

Once she had established a right to marriage, Bonauto went after the Defense of Marriage Act, which denied legally married gay couples the federal benefits that straight couples could have. Barney Frank, the former Massachusetts congressman who is openly gay, said the move had sealed Bonauto’s reputation as a “first-rate lawyer and a first-rate strategist” who built on one victory after another.

Referring to her in terms of the Supreme Court justice who made history fighting racial discrimination: “She’s our Thurgood Marshall.”