The station signed on the air on January 3, 1970. It was originally owned by Warren Adler along with WHAG radio in Halfway (AM 1410 and FM 96.7, now WDLD). WHAG-TV's original analog transmitter was to be on top of the Hagerstown Motor Inn (now the Alexander House) but was rejected due to structural incompatibility. A site on Fairview Mountain would become the location of the analog signal on UHF channel 25. Regional Broadcasting Company wanted the station to affiliate with ABC (which was the number one network at the time) but had to join NBC, which was the number three network. Adler Communications sold WHAG-TV to Sheldon and Samuel Magazine of Washington D.C. in 1973. The Magazine Brothers then sold it to local aviation pioneer Richard Henson in 1977. Henson then sold the station to Great Trails Broadcasting in 1981. Great Trails exited broadcasting and sold WHAG along with 2 of its stations—WFFT-TV in Fort Wayne, Indiana and KSVI in Billings, Montana to Quorum Broadcasting in 1998 for $65 million.
On September 8, 2003, Nexstar Broadcasting Group announced that it would acquire Quorum Broadcasting and its stations (including WHAG-TV) for $230 million. The sale was completed on December 31, 2003.
WHAG-TV shut down its analog signal, over UHF channel 25, on June 12, 2009, the official date in which full-power television stations in the United States transitioned from analog to digital broadcasts under federal mandate. The station's digital signal relocated from its pre-transition UHF channel 55, which was among the high band UHF channels (52-69) that were removed from broadcasting use as a result of the transition, to UHF channel 26. Through the use of PSIP, digital television receivers display the station's virtual channel as its former UHF analog channel 25.
Beyond NBC programming, WHAG's line-up has been stable and consistent. It does not carry the network's Early Today. Long blocks of infomercials have been shown on weekends since the early-1990s. Around September 2006, infomercials began appearing on weekdays during the daytime. WHAG-TV is one of the few NBC affiliates that does not air the fourth hour of Today.
Right from the start, WHAG began offering local newscasts with The Valley News which aired weeknights at 6, 7, and 11. The original anchors were Bob Witt with news, Glenn Presgraves with sports, and Bill Wolfinger forecasting the weather. Bill Wolfinger also did a Saturday night horror movie show where he would be in costume similar to Lon Chaney. The news department expanded in 1972 to include weekend evening broadcasts at 11 that totaled six hours of local news per week. By the year 2000, news content increased to over 22 hours of broadcasts per week. In 1997, WHAG added a microwave truck allowing the transmitting of live breaking news from the viewing area. On February 12, 2010, WHAG dropped the "NBC 25" branding for "WHAG" and switched its news branding from "NBC 25 News" to "WHAG News". This also happened at the same time a new set, music, and graphics package was launched. The station operates a bureau on East Patrick Street (MD 144) in Frederick.
On August 30, 2010, WHAG added a half hour to its weekday noon and 5 p.m. newscasts. Until this point unlike most NBC affiliates in the Eastern Time Zone, the station had not aired a broadcast weeknights at 5:30. It still does not offer a full two-hour weekday morning show. There is now a half hour broadcast seen Monday through Saturday nights at 7. On weekends, an hour-long morning show at 6 as well as a half hour Sunday morning broadcast at 9 were added. In addition, a Northern Virginia Bureau covering Leesburg, Berryville, and Winchester was opened. Although not a full news department, this is now the second local news operation established in those areas after TV3 Winchester launched back on March 5, 2007. All of the preceding changes required the expansion of WHAG's personnel.
On October 21, 2013, WHAG began broadcasting its local newscasts in high definition, with a new news set, HD cameras and forecasting equipment.
The station's news department is mainly known as a training ground with few permanent personalities, as reporters often come fresh out of college and serve as "one man band" personnel that shoot, write, and edit their own stories. They often move on after a year or two to larger markets or other communications opportunities.
Recently, it has been added to the Dish Network lineup of local offerings and is available to subscribers that currently receive the Washington D.C. market locals. WHAG had also been seen on Dish as the default NBC affiliate for the Salisbury television market, as that market did not have an NBC affiliate of its own until June 2014, when WRDE-LD in nearby Rehoboth Beach, Delaware switched its affiliation to NBC.