These days, women in the U.S. Marines have reached all kinds of milestones. Last week, for example, the New York Times reported on how Lieutenant Marina A. Hierl, 24 — one of only two women to have passed the 13-week Marines Corps’ Infantry Officer Course at Quantico, Va. — is adapting to her new job as the first woman in the Marines to lead an infantry platoon.

This milestone comes about a century after another important one: the day Opha May Jacob Johnson, then 40, became the first woman ever sworn into the Marines Corps. Monday marks the centennial of her Aug. 13, 1918, enlistment.

Some of the details about Johnson’s life are easy to track down: Born in Kokomo, Ind., on May 4, 1878, she was raised in D.C. and graduated second in her class from Wood’s Commercial Business College. On Dec. 20, 1898, she married a musician named Victor Hugo Johnson. Before she was in the Marines, she worked for 14 years in the civil service in the Interstate Commerce Department as Clerk to the Quartermaster General.

At some point on the job, something changed — and after that, the details get a little blurry.

Opha May Johnson is thought to have been invited to join the Marines, according to Nancy Wilt, the Women Marines Association member who started researching Johnson shortly joining the organization in 2005. “I felt there was very little history of women Marines preserved,” she says. The story also resonated with her personally. Like Johnson, she worked behind the scenes in food services and was one of two female Marine officers on a base in Millington, Tenn.

But exactly why Johnson joined and what she did on the job is still not clear, because those attempting to tell her story have not found diaries or other personal records of Johnson’s feelings. In the absence of such documents, Wilt and her colleagues have worked mostly from newspaper articles, courthouse records, performance evaluations and other documents about her in the U.S. Marine Corps archives, which suggest that her top marks prompted the invite to enlist. “She was a whiz-bang,” says Wilt of Johnson’s apparently stellar administrative skills.

On the enlistment paper she signed — showing that she passed a physical exam on Aug. 12, and enlisted on Aug. 13 — the male pronouns that were the default have been crossed out.