In 1899, the Astronomical and Astrophysical Society of America (later the American Astronomical Society) was established. McLeod was an early member, and C.A. Chant and J.S. Plaskett began attending AAS meetings from 1906 and 1907, respectively. Both would be prominent in the society. Plaskett, particularly, brought a number of his Dominion Observatory (and later Dominion Astrophysical Observatory) colleagues to the meetings. Until well into the 1950s, Canadians maintained a high profile in the AAS, holding offices and committee memberships, while presenting numerous papers at annual meetings. Several AAS meetings, beginning with Ottawa in 1911, were held in Canadian cities.

Until after World War II, the number of Canadian professional astronomers was still very small. The RASC provided professionals with an organizational outlet, in a way that the Royal Society never had. Professionals were always prominent at the national level and in the local centres. C.A. Chant, who trained most of the pre-war astronomers at Toronto, edited the RASC's Journal and Observers Handbook for decades. Indeed, the RASC was unique amongst national astronomical societies, bringing amateurs and professionals together under one roof.

After World War II, Canadian astronomy expanded slowly. Besides the two national optical observatories, the Dominion and Dominion Astrophysical Observatories, radio astronomy installations appeared in1960 with the DO's station, the Dominion Radio Astrophysical Observatory, and the National Research Council's Algonquin Radio Observatory. The NRC also supported research in laboratory astrophysics and meteor science. The University of Toronto's David Dunlap Observatory, which had opened in 1935 with the world's second-largest reflector, was the only significant university facility and the only centre for graduate studies in astronomy. In those days, Canadian astronomers were most likely to meet their colleagues at AAS meetings or at meetings of the Astronomical Society of the Pacific or the International Union of Radio Science (URSI).

George Ellery Hale, the founder of the Yerkes and Mount Wilson Observatories (and the father of the Mount Palomar Observatory), had an unwitting hand in the organization of Canadian astronomers. J.S. Plaskett was one of Hale's great admirers and when the latter formed the International Astronomical Union, Plaskett ensured that Canada was an early adhering nation. From the early 1920s, Canada, through the Dominion Observatory, was an active member, with its astronomers on several IAU commissions. Each adhering nation had a National Committee, though in practice, committee work was handled by the DO. When Carlyle Beals was Dominion Astronomer in the 1950s and 1960s, he envisioned a broader role for the Canadian National Committee (CNC). With government funding, CNC meetings began to attract more than just committee members: a prominent astronomer, often an American, would give an address, along with a few papers by Canadian astronomers. Not surprisingly, government astronomers were most likely to attend.

The RASC had grown larger, and was beginning to hold annual, national meetings of its own, but these meetings did not focus upon professional contributions like the AAS meetings. Many Canadian astronomers published in the pages of the RASC's Journal, but it was not the journal of record for most professionals. When the 1960s arrived, university programmes in science expanded rapidly, and astronomy groups and departments appeared across the country. University-based astronomers, particularly younger ones in smaller centres, did not have the financial means to attend CNC meetings regularly. The lack of regular contact and a forum for debate over future directions of Canadian astronomy were factors contributing to the cancellation of the Queen Elizabeth II telescope project in 1968. A further stimulus for organizing came when institutional players were reduced in number. In 1970, the federal government closed the Dominion Observatory and transferred its staff and facilities to the National Research Council (NRC).

The NRC had, for many years, formed Associate committees with members drawn from the Council's laboratories, industry and academia to focus upon specific research areas. With the consolidation of government astronomy, an Associate Committee for Astronomy was now created; it had, effectively, the same membership as the CNC. A sub-committee of this group, composed of Vic Hughes of Queen's University, Michael Ovenden of the University of British Columbia and Robert Roeder of the University of Toronto, proposed that a professional society should be formed. It estimated such a society might initially attract between 100 and 150 members (which, in the event, it did).(Charter members) At its meeting in Victoria in May 1971, the Associate Committee/CNC accepted this recommendation and elected a council of seven. Helen Hogg, of Toronto, was chosen the first president, with Peter Millman, of the NRC, as the inaugural secretary. This was considered the founding meeting. The first meeting followed at the University of Toronto that November.

During the early years, the Canadian Astronomical Society or CAS (its bilingual acronym CASCA came later) worked to build up its credibility amongst its own clientele. Early members realized the importance of informing and advising government on the profession's needs, but a mechanism for such advice emerged only later. CASCA had no official journal - though it reported its activities through the Journal of the Royal Astronomical Society of Canada - and informed its membership through a newsletter. Alan Batten, of the DAO, CASCA's second president, began issuing occasional leaflets. David DuPuy, at St Mary's, suggested a regular newsletter. Cassiopeia was thus established, with DuPuy and Jack Heard of the DDO as co-editors. Cassiopeia went through over 100 quarterly issues, named for the solstices and equinoxes, until it was replaced by an electronic version.

Before the Society formed, Canadian astronomers were very active in the AAS. Once CASCA grew in membership and complexity, Canadians put more of their energy into their own organization, though many still attend the American meetings. CASCA's organization and meeting format developed along lines familiar to AAS members. Paper sessions became larger and more formal, compared with the early CNC days. Plenary and poster sessions were introduced, along with a series of awards and a range of committees. Unlike its American counterpart, CASCA holds only one annual meeting, moving back and forth across Canada, though the Board of Directors (as the council came to be called after incorporation in 1983) met more often.

During a period of retrenchment, the National Research Council terminated its associate committees, including that of astronomy. The Canadian National Committee of the IAU, however, remained intimately associated with CASCA; the Board acts as the CNC. CASCA's early hopes of providing advice to government bore fruit with the organization of an Advisory Committee to the Herzberg Institute of Astrophysics, NRC's astronomy division. The culmination of CASCA's sounding of the profession to set priorities can be seen in its recent participation in the work of the Long-Range Planning Panel.