WMEX began broadcasting in 1934, where it is said to have broadcast remotely a live Billie Holiday Boston performance.

In 1941 jazz scholar and critic Nat Hentoff launched a jazz program that lasted through the decade. Organist John Kiley, Red Sox organist for 35 years, would take listener requests and play the tunes live right there in the studio. In 1957, following an ownership change, WMEX changed  to a rock-and-roll format, one of the first in the US to do so, and began airing the legendary Arnie "Woo Woo" Ginsburg, whose nightly rock-and-roll broadcasts on 1600 WBOS were already very popular.

With other personalities such as Emperor Fenway, Melvin X Melvin and Dandy Dan Donovan, the station was the go-to for Boston youth. Ginsburg frequently ran record hops in Legion halls and community centers for local high-schoolers and famously created the "Ginsburger" for sponsor Adventure Car Hop.

In 1967, the popular WRKO went Top-40, but in Dirty Old Boston, WMEX was the real deal. For example in later 1967 while 'RKO aired the Doors "Light My Fire" 45 edition, 'MEX played the album version, complete with Ray Manzarek's organ solo.

This endeared music enthusiasts to the station and until the advent of Underground FM radio and the rebirth of WBCN in 1968, WMEX was king. 

In the late 60s, WMEX received a power upgrade to 50,000 watts daytime, still with 5000 watts at night. Station engineers had to constantly adjust the phasing network as tides in the Neponset River would wreak havoc on the station's directional pattern. However, the saltwater marsh area provided the station with an excellent coastal signal. While the night signal could not be heard clearly inland at many Boston suburban locations (especially in the growing and affluent western and southwestern suburbs), the station's nighttime transmissions were heard very clearly in Boston's neighborhoods and working class North Shore areas like Lynn and Salem. The saltwater path nighttime transmissions also reached up Nova Scotia and Labrador, gaining the station a wider audience.

By the late 60s, WMEX, lacking the nighttime 50K watts, had a tough time competing with WRKO which had the juice day and night. But WBZ veteran Dick Summer and (later) V-66 launcher John Garabedian were able to introduce an expanded playlist featuring what were known as Progressive Rock album cuts, eschewing the still-dominant Top-40 mentality, by that time the laughing stock of the Hipoisie.

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