WTNH first went on the air on June 15, 1948, as WNHC-TV, originally broadcasting on channel 6. The station was founded by the Elm City Broadcasting Corporation, owners of WNHC radio (1340 AM, now WYBC; and 99.1 FM, now WPLR). The station is Connecticut's oldest television outlet and the second-oldest in the New England region (WBZ-TV in Boston signed on less than a week earlier).

WNHC-TV was originally an affiliate of the DuMont Television Network, and claims to have been the first full-time station of that short-lived network. The station broadcast from WNHC radio's building on Chapel Street in downtown New Haven. However, with no studio facilities of its own, it could not produce local programming. For a time, WNHC-TV simply rebroadcast the signal of DuMont's New York City flagship, WABD (now WNYW). In October 1948 the station added CBS programming to its schedule, and additional secondary affiliations with NBC and ABC followed a year later. The station was the first station in the country to use videotape for local programming and one of the first to broadcast in color.

When the Federal Communications Commission (FCC)'s Sixth Report and Order ended the four-year freeze on television construction permit awards in 1952, it also reorganized channel allocations to alleviate interference issues. As a result, WNHC-TV changed frequencies and moved to channel 8 in December 1953. The next year, the FCC collapsed New Haven and Hartford into a single market. WNHC-TV shared some CBS programming with New Britain's WKNB-TV (channel 30, now WVIT) until 1955, since WKNB's signal was not strong enough to cover New Haven at the time.

In 1956, the WNHC stations were bought by Triangle Publications of Philadelphia. Also that same year, WNHC-TV lost its CBS affiliation when that network purchased WGTH-TV in Hartford (channel 18, later WHCT and now WUVN). This left channel 8 as a sole ABC affiliate, although it shared ABC programming with WATR-TV (channel 20, now WCCT-TV) in nearby Waterbury until 1966.

Until the original WTIC-TV (channel 3, now WFSB) signed on from Hartford in September 1957, WNHC-TV was the only station on the VHF dial in Connecticut. Many viewers northeast of Hartford used outdoor antennas to get spotty reception of CBS and NBC programs from Boston, and those southwest of Hartford with outdoor TV antennas got equally spotty reception from their respective New York flagship stations. By contrast, most of Connecticut got a clear picture and pitch-perfect sound from channel 8.

Later years

Triangle was forced to sell its broadcast outlets in 1970 after then-Pennsylvania Governor Milton J. Shapp complained the company had used its Pennsylvania stations in a smear campaign against him. The WNHC stations were among the first batch to be sold, going to Capital Cities Communications, along with sister stations in Philadelphia and Fresno, California in a deal finalized in 1971. However, Capital Cities could not keep the radio stations because of the FCC's then-restrictions on ownership, resulting in WNHC-AM-FM being spun off to separate third parties. WNHC-TV changed its call letters to the current WTNH-TV in April 1971, not long after Capital Cities officially took over (the station dropped the -TV suffix from its calls in 1985, but continued to call itself "WTNH-TV" on-air well into the 1990s).

Capital Cities bought ABC in 1986 in a deal that stunned the broadcast industry. But the merged company was unable to keep WTNH due to a significant signal overlap with ABC's flagship station, WABC-TV in New York City. Like the other major stations in Connecticut, WTNH's city-grade signal reaches Fairfield County, which is part of the New York City market. It also provides at least grade B coverage to most of Long Island. At the time, the FCC normally did not allow common ownership of two stations with overlapping coverage areas, and would not even consider granting a waiver for a city-grade overlap (the FCC began allowing common ownership of two stations with overlapping coverage areas in 2000). As a result, WTNH was spun off to Cook Inlet Television Partners, a subsidiary of Cook Inlet Region, Inc., an Alaska Native Regional Corporation. During the mid-1980s, the syndicated Sally Jessy Raphael talk show originated from the WTNH studios in New Haven, until the show moved to New York City.

Cook Inlet sold WTNH to LIN Television in 1994. When a new UHF independent station in New Haven, WTVU (channel 59, now WCTX) signed on in 1995, WTNH began operating the station through a local marketing agreement (LMA). In 2001, LIN TV bought WCTX outright. On May 18, 2007, the company announced that it was exploring strategic alternatives that could have resulted in the sale of the company. On March 21, 2014, Media General announced that it would purchase LIN Media and its stations, including WTNH and WCTX, in a $1.6 billion merger. The merger was completed on December 19.

Analog-to-digital conversion