From 1974 to 1988, the call letters WBMX were held by 102.7 FM, licensed to Oak Park, Illinois (Now Urban AC station WVAZ). On December 4, 2017, the call letters were transferred from Boston (where they had been held by 104.1 FM since 2009) to 104.3 FM in Chicago.
The station began broadcasting in September 1953, holding the call sign WSEL. The station had an ERP of 40,000 watts, and its transmitter was located atop Chicago's Randolph Tower. WSEL was owned by Chicago Skyway Broadcasting Company.
A previous station in Chicago had briefly operated on 104.3 MHz in 1949. WCFL-FM, owned by the Chicago Federation of Labor, broadcast from 3 to 9 p.m. as a 400-watt simulcast of WCFL, but the owners surrendered the station's license, as it was not profitable.
In 1958, WSEL's transmitter was moved to the Willoughby Tower at 8 South Michigan Avenue. In 1960, the station was sold to Plough Broadcasting for $50,000, and it was taken silent.
In 1971, the station's transmitter was moved to the Prudential Building, and its ERP was reduced to 14,100 watts.
In February 1977, the station's call sign was changed to WJEZ. The station adopted a "beautiful country" format, playing easy listening country music, with large amount of instrumentals, in an approach patterned on the beautiful music format.
In early August 1984, the station adopted an oldies format as "Magic 104" and its call sign was changed to WJMK.Dick Biondi was the first disc jockey heard on "Magic 104". Ron Britain was also one of WJMK's original DJs. Initially, "Magic 104" included a few currents in its playlist, but by early 1985, all songs from the current decade were dropped, with the station playing music from the 1950s, 1960s, and early 1970s.
In 1987, the station's transmitter was moved to the Sears Tower, and its ERP was reduced to 4,100 watts.
In 1991, the station's moniker was changed to "Oldies 104.3". In early September 1993, John Records Landecker joined WJMK as morning drive DJ, remaining with the station until 2003.
In 1996, Infinity Broadcasting was purchased by the parent company of CBS.
In 1999, with competition from the new Jammin' Oldies format of WUBT, WJMK increased the amount of 1970s songs played on the station while reducing the amount of pre-1964 songs played. By 2002, the station would replace the All Request Saturday Night oldies show with a 1970s and early 80s program.
On February 15, 2002, WJMK returned to its former moniker, "Magic 104.3", and its playlist was shifted to include more 1980s music, while further reducing the music played from the '50s and early '60s. In July 2003, the station once again changed monikers, going back to "Oldies 104.3" and its playlist was refocused on music of the '60s and '70s. In 2004, the station dropped the "Oldies" moniker and became known simply as "104.3 WJMK" with the slogan "The Greatest Hits of the 60s and 70s".
On June 3, 2005, at 4 p.m., WJMK switched to an adult hits format known as "Jack FM" at the same time veteran oldies station WCBS-FM in New York City made the same switch. The station had an 1980s centric playlist, along with some titles from the 1960s, 1970s, 1990s, and 2000s. It usually had no live DJs and instead used sarcastic remarks voiced by Howard Cogan during breaks.
Though WJMK's previous oldies format continued to be streamed online and on the WJMK's second HD Radio subchannel, complaints about WJMK's switch were numerous. In July 2006, in a cost-cutting move by CBS Radio, the entire DJ staff of WJMK-HD2 was laid off. Shortly thereafter, 94.7 WZZN, which had recently switched to an oldies format, hired several of WJMK's former airstaff.
With a format change on WCKG from hot talk to adult contemporary, Steve Dahl and Buzz Kilman moved to WJMK to host mornings on November 5, 2007. Dahl was dismissed on December 5, 2008. With the exception of Dahl and Kilman, Chicago's Jack FM had no live personalities.
WJMK's ratings plummeted after the switch to Jack FM, and the station saw further ratings erosion when Bonneville International debuted "Rewind 100.3" (a mostly 1980s-based format) on rival WILV in June 2010.
Chicago radio personalities Ed Volkman and Joe "Bohannon" Colborn (Eddie and JoBo) hosted the station's morning show, along with Gary Spears in middays, Bo Reynolds in afternoon drive time and George McFly heard in the evening. Weekend hosts included Tommy Edwards, Ken Cocker, and John Calhoun.
Eddie and JoBo were released on December 6, 2012, with the station citing low ratings as the main factor. Mornings were then hosted by Dave Fogel, formerly of WLS-FM. Tommy Edwards retired from radio on September 12, 2014. The rest of the station's final airstaff included Brian Peck in middays and Jeffrey T. Mason in afternoon drive.
In its last year, WJMK primarily played music from the 1970s and 1980s.
On February 2, 2017, CBS Radio announced it would merge with Entercom. The merger was approved on November 9, 2017, and was consummated on the 17th.