She was also known for a short time as "Mrs. Sherlock Holmes", starting with her work solving the cold case of Ruth Cruger who disappeared in New York in 1917.

[2] Dean Ashley of the law school was impressed with her abilities and urged her to attend evening sessions, so that she completed the three-year course in two years and graduated ranked seventh in her class.

[3] She graduated with a Bachelor of Laws in 1903[4] and, after spending one year with the Legal Aid Society, was admitted to the bar in 1904[1] and eventually made the decision to practice regularly.

Quackenbos explained in The New York Times after a successful case that she started it to provide working people with "St. Regis law at Mills Hotel Prices, and such other assistance as they may need in the redressing of wrongs at a cost within their means."

She went on to say, "My idea in establishing the firm was to demonstrate that a legal bureau for the aid of the poor could be operated at a scale of prices within their reach and to their great benefit, and I think this has been done.

"[5] One of her best-known cases was that of Mrs. Antoinette Tolla, a woman from Kingsland, New Jersey, who was accused of murdering Joseph Sonta and sentenced to death.

"[6] On March 9, three days before Mrs. Tolla was to hang, Quackenbos, after a week's effort, induced the board of pardons of New Jersey to commute the death sentence to seven and one-half years imprisonment.

[1][7] During the early years of the People's Law Firm, Quackenbos was approached by several clients that wanted assistance finding relatives or friends that had gone South and then disappeared completely.

[3] In the spring of 1907, while at the Department of Justice, Quackenbos continued to investigate cases of immigrant laborers held in peonage across Southern states.

On June 4, 1907, he wrote to Secretary of State Elihu Root requesting that Quackenbos investigate the conditions of Italian laborers in the Delta.

[10] Quackenbos arrived at Sunnyside Plantation across the river from Greenville, Mississippi in the town of Lake Village, Arkansas in July 1907 to investigate the allegations.

[11] While at Sunnyside Percy arranged for her notes to be stolen from her hotel room and then "recovered" by a close associate – sending the message to Quackenbos that she could not touch him.

She took the case pro bono, and after interviewing several Harlem residents and deciphering a blurred message on a blotter,[16] Humiston determined to search the basement of suspect Alfredo Cocchi.

[15] Cruger then accused the NYPD of negligence, and a subsequent investigation revealed that there had been a longstanding kickback scheme between Cocchi and the local police.

Because of the case and the resulting public criticism, Humiston was named a special investigator to the New York City Police Department, charged with tracing missing girls, in July 1917.

As a result of the case Humiston incorporated the Morality League of America, an organization that was founded to gather and disseminate facts which will impress the public with the dangers to women and girls throughout the United States...and to investigate and report to the authorities for prosecution complaints regarding immoral conditions which may exist throughout New York and the United States and urge the passage of legislation necessary to promote the purposes of the organization.