Beginning in nightclubs and on tour (for years Leno was making 300 personal appearances annually), Leno made a few early stabs at television and acted in several films ("American Hot Wax" 1978, "Americathlon" 1979). He did not really hit his stride, though, until the mid-1980s, when numerous appearances on late-night TV (especially "The David Letterman Show") gained him a niche as an occasional replacement for Johnny Carson on "The Tonight Show". After Leno's quick wit and easy manner managed a more-than-fair share in the ratings, he was made the show's permanent guest host in 1988 and chosen to be Carson's successor in 1992.

Following in Carson's well-established footsteps was certain to prove a daunting undertaking, and there was an initial slight ratings slip as well as some backstage problems with former executive producer Helen Gorman Kushnick. On the whole, though, Leno managed the transition with the good-humored grace viewers had long since come to expect from him. When David Letterman jumped ship from NBC to CBS after being passed over for the "Tonight Show" hosting gig, Leno found himself facing much more formidable competition than he or Carson (who tended to favor Letterman) had encountered, sparking a hotly contested battle for viewers. The ratings war waged on neck-and-neck throughout the 1990s, but eventually, while critics tended to favor Letterman's edgier, snarkier show, Leno's broader, middle-of-the-road populist approach gradually emerged as the consistent ratings winner, bolstered by NBC's stronger prime-time lineup lead-in and promotional platform. However, in exchange for mainstream popularity, Leno—a notorious workhorse who took few days off and usually performed in clubs and concert halls when he did—seemed to lose much of his own distinctive brand of comedic bite with each successive season. In 2004, Leno announced his planned retirement from "The Tonight Show" in 2009 and planned to hand his hosting duties over to his NBC late night cohort Conan O'Brien.