The station began broadcasting during the daylight hours from the Hermant Building at 21 Dundas Square on 28 October 1945 on 1050 kilohertz with 1,000 watts of power. Operated by York Broadcasters, the station was owned by entrepreneur Jack Q'Part, who saw it as providing an excellent avenue for marketing his patent medicine products. However, many feel that the station really didn't find its niche until ten years later when Allan Waters, president of a pharmaceutical company owned by Part, purchased the radio station in December 1954. His first task was to change the station's format to rock and roll. Waters was inspired by the success of stations owned by Todd Storz in the United States that were successfully reaching younger audiences through the repetitive play of new records. Waters altered the programming day to broadcast 24 hours a day, and he moved the station's transmitter, increasing power to 10,000 watts. On 27 May 1957, the station unveiled its Top 40 music format. It took two or three years before advertisers became interested in the station, as the Top 40 format was unproven in the Canadian marketplace. During this time the station moved twice, to 225 Mutual Street and 250 Adelaide Street West, before settling in 1959 at 1331 Yonge Street, where it remained at the beginning of the 21st century.

     Among the most significant developments occurring during the 1950s was the establishment of the CHUM chart, a Top 50 list of hit singles issued weekly. The first CHUM chart appeared on 27 May 1957, with Elvis Presley's "All Shook Up" assuming the number one position, ahead of Pat Boone's "Love Letters in the Sand." CHUM's chart remained a Top 50 list until 1968 when it was reduced to 30 songs. It remained in that format until 1986. In 1984 the chart was converted to a television format with a program that aired weekly on CITY­ TV, a local Toronto station owned by the CHUM radio group. The TV program incorporated (newly emerging) music videos. Available for listeners at local record stores and published in newspapers, the CHUM chart served as an important marketing tool to boost record sales, particularly of domestic acts such as Lighthouse and Blood, Sweat and Tears. (A complete list of CHUM charts is available at the station's website, www.1o5ochum.com.)

     In the 1960s the station's prominence as Canada's strongest radio station continued. This was due largely to the emergence of rock and roll, the station's relentless promotional activities, and its roster of popular disc jockeys. Before heading off on his long stint with American Bandstand, Dick Clark hosted a series of shows broadcast on CHUM. Other popular disc jockeys of the era include Dave Johnson, Mike Darrow, Bob Laine, Bob McAdorey, and Duff Roman. One of the biggest celebrities was "Jungle" Jay Nelson, who, after working as the host of an eclectic children's program on WKBW in Buffalo, New York, was recruited by CHUM in 1963 to replace the popular morning host Al Boliska.

     On 1 September 1963 CHUM-FM opened at 104.5 megahertz with 18,000 watts of power, broadcast from the same Yonge Street location. Its program content was different from that of the AM station, as CHUM-FM broadcast classical and fine arts programming. In 1964 the station upgraded to 50,000 watts and started transmitting from Clarkston, Ontario. This transformed CHUM from a Toronto area radio station to an outlet that reached most of the population of Ontario and some U.S. cities on the Eastern seaboard, as well. By 1967 the station traded under the name of CHUM Limited on the Toronto Stock Exchange; and CHUM-FM began broadcasting 24 hours a day.

     Owing to failing ratings and the superiority of FM radio by the 1980s, CHUM-AM underwent a number of format changes, moving away from the Top 40 format (Top 40 was moved, along with the CHUM chart, to the FM station) toward a format of "favorites of yesterday and today" in 1986. (That same year, the station was knocked off the air when the transmitter tower was sabotaged. There were rumors that this act was motivated by the unpopular change in musical format, but in fact one of the perpetrators had an ongoing grudge with the station because the tower interfered with his television reception.) The 1986 format change was followed by a change to soft rock in 1988. This strategy was also short­-lived, and in 1989 the station adopted an all-oldies broadcast format. Previously, CHUM broadcast Toronto Argonaut football in the Canadian Football League, as well as the Toronto Blue Jays baseball games.

     The station also featured a number of prominent personalities over the years. John Roberts, later an anchorman working with CBS television, began his career as night-time disc jockey J.D. Roberts on CHUM before moving to work on CITY-TV. Other prominent personalities formerly with CHUM include Bob McGee, Dick Smyth, Tom Rivers, and Allan Slaight, a CHUM program director and general manager in the 1960s and later president of the Standard Broadcasting Network, which owns a number of radio stations across Canada.

     On 7 May 2000, 1050 CHUM signed off its popular oldies format and began an experiment in all-sports radio using a number of CHUM-owned radio stations across Canada. 1050 CHUM became the anchor station in the fledgling TEAM sports network. On 27 August 2002, the station abruptly dropped the all-sports format, returning to a mix of hits from the 1950s, 1960s, and 1970s. On 29 November 2002, Allan Waters stepped down as chairman and president of the CHUM group, which now owns a number of television and radio outlets across Canada. His two sons, Jim and Ron, continued to hold other executive posts with the company.