In 1939, she was one of the featured aviators in an air show held in Montreal in honor of King George VI and heir presumptive, Elizabeth. After having 13 suffrage bills defeated between 1922 and 1939, Quebec's feminists worked that year to convince Premier Adélard Godbout to keep his campaign promise to reintroduce the measure. On 9 April 1940, he personally introduced "Bill No. 18" proposing women's suffrage at the provincial level. The bill passed with a vote of 67 in favour and 9 against on the third reading on 18 April. Gaining approval on 25 April from the Legislative Council, feminists saw the vote as leverage to gain other reforms. The following year, Langstaff, Leona Bell (wife of Leslie Bell) and Elizabeth C. Monk submitted a plea asking support for women's right to practice law to the Quebec Bar Association. In a vote of 12 in favour and 11 against, the bar agreed to allow it as long as the legislature approved a bill in the sitting session. On 29 April 1941, the legislature passed the bill and in early 1942, Monk, Constance G. Short, and Marcelle Hemond were the first women admitted to the bar. Impacting Langstaff, the new law required that candidates have a Bachelor of Arts degree for admission to the bar and she was not prepared to study for another degree. Langstaff retired in 1965, having spent six decades at Phillips & Vineberg. Langstaff died on 29 June 1975 in Montreal, after a lengthy illness, and was buried at Cornwall, Ontario. Her papers were collected by her daughter and placed in the archival holdings of the Faculty of Law at McGill University. In 1988, McGill's Faculty of Law inaugurated the "Annie MacDonald Langstaff Workshops" in her honour to provide a forum for academics, judges, lawyers, and community activists to present scholarly research and practical insights on issues relating to women and the law. Two other named lectures, the "Margot E. Halpenny Memorial Lecture" and the "Patricia Allen Memorial Lecture" are given each year as part of the series, in addition to three or four additional presentations. In 2006, more than 90 years after she had first been refused admission to the bar, Langstaff was posthumously admitted to the "Barreau de Montréal" and awarded the bar's rarely-given Medal of Honour. As by that time, her daughter, who had become a nun in the Sisters of Holy Cross, had already died, the medal was accepted by Davies Ward Phillips & Vineberg, the successor firm to her former employer.