The station began broadcasting experimentally on February 2, 1940, as W9XEN. Shortly thereafter, the station would be licensed as W51C, broadcasting at 45.1 MHz. It was one of the first FM stations in the United States, and is the country's oldest FM station still in operation. The station was owned by the Chicago based radio/television manufacturer Zenith Radio Corporation. Its transmitter was located atop the Field Building.
In 1943, the station's call sign was changed to WWZR. In 1946, the station's call sign was changed to WEFM, and it began broadcasting on 98.5 MHz, while continuing to broadcast at 45.1 MHz. WEFM's call letters stood for the initials of Zenith Radio Corporation president Eugene F. McDonald. In 1947, the station's frequency was changed to 99.5 MHz. On June 1, 1961, WEFM became the second station in the United States to broadcast in FM stereo. In 1972, its transmitter was moved to the John Hancock Center.
From 1940, when the station began broadcasting, until February 1978 the station aired a classical music format. Few advertisements were aired, and until 1966 the only advertisements were for Zenith products. In 1966, the station began to sell advertising time, though commercials were limited to five minutes per hour and the ads had to be compatible with WEFM's classical music format.
In the early 1970s Zenith agreed to sell WEFM to General Cinema Corporation, which intended to change the station's call letters to WICV (pseudo-Roman numerals for 99.5) and institute a rock format. Litigation delayed the sale and format change until February 1978. As part of the settlement to allow the station's sale, WEFM's classical music library was donated to WNIB and WBEZ. General Cinema converted the station to a top 40 format with program director Brian White and afternoon drive personality Don Cox, but decided not to change the call sign. The station was branded "We-FM" and initially broadcast from the studios used by the Zenith classical music format at 120 West Madison street in the Chicago loop. General Cinema moved studios to the 13th floor of the Hancock Center at 875 N. Michigan Ave in 1980, where the transmitter resides on the 93rd floor. The station leased a 67 kHz subcarrier to the Physicians Radio Network, a news service for medical doctors. In early 1981, the station adopted a MOR format, with programming from the syndicated Schulke II package.
In 1982, the station was purchased by First Media Corporation for $9.2 million. On February 6, 1982, the station adopted a country music format, branded "US-99", and its call sign was changed to WUSN on February 25, 1982. The station's initial promotion was that four songs would be played before any commercial break ensued, and that $25,000 would be given to the first person to call if the guarantee wasn't fulfilled. Within the first week, two mistakes were noticed by listeners and $50,000 was given away.
Lee Logan was hired as program director from KFMK in Houston, remaining with the station until 1987, when he departed for KLAC in Los Angeles. From 1982 to 1985, Don Wade was the station's morning host. Wade was briefly midday host on the station, before moving to WLS. Shock-Jock Gary Dee replaced Wade as morning host in 1985, but was fired a year later.
The station's initial country music competitors in Chicago were 670 WMAQ, 104.3 WJEZ, and 1160 WJJD, which switched to the adult standardsMusic of Your Life format within weeks of "US-99"'s debut. In years when the station lacked major local competition, it has ranked as the nation's most listened to country station.
On August 8, 2016, WUSN rebranded slightly as "US?99", dropping the .5 from their moniker and unveiling a new logo and slogan, "Chicago's Hottest Country". The traditional star seen in their logo was changed in the new version to the six-pointed variety represented in the acclaimed Flag of Chicago, with the logo coloring following suit using the flag's light blue and red.
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.
In 2005, WUSN host Lisa Dent won the Country Music Association's Major Market Personality of the Year award.
In 2010 and 2011, the station won Country Radio Broadcasters/Country Aircheck Awards for Station of the Year for a Major Market; the Lisa Dent and Ramblin' Ray Show for Major Market Morning Show and Marci Braun (weeknight host/MD) for Major Market MD.
In 2010 and 2015, Lisa Dent and Ramblin' Ray Stevens won the Country Music Association Major Market Personality of the Year for the Lisa Dent and Ramblin' Ray Morning Show.
On April 13, 2001, a memo from WUSN management asking on-air station employees to attend the George Strait Country Music Festival on May 26, 2001 at their own expense and "work the crowd" on behalf of the station was leaked to Robert Feder's media column for the Chicago Sun-Times.