In 1991, W140 series replaced the W126 line in the Mercedes-Benz lineup, and the first production model of the latest S-Class rolled off the assembly line on August 6 of that year. As with its predecessor, the W140 was the first of the "next generation" of Mercedes-Benz models to feature the company's new design theme. Following S-Class tradition, the car was available in two wheelbase lengths along with the shorter-wheelbase W140 coupe. Production totaled 406,532 units, the reduced total reflecting the inroads made by the Lexus LS into the key US export market.

The W140 S-Class is often known as the last Mercedes-Benz to be "overengineered," a company trait which was costing Mercedes-Benz in product delays and overbudgeting. For the consumer, the W140 cost a considerable 25% more than the preceding W126. Intense pressure from competitors such as Lexus and Infiniti led to the addition of more features and options as to set the W140 apart from the rest and to justify the higher price. The W140 introduced new innovations such as double-pane window glazing, self-closing boot lid and doors, electric windows with a jam-protection feature (lowering back down when encountering an obstruction), rear-parking markers (which appeared on the rear wings when in reverse), and a heating system which emitted warm air even after the engine was turned off.

In 1994, Mercedes-Benz model nomenclature was completely rationalized, with the SE/SEL/SEC cars becoming the S-Class and alphanumerical designations inverted (e.g. the both 500SE and 500SEL became S500 regardless of wheelbase length). In 1997, following the new Mercedes-Benz tradition of mid-life facelifts (first with the W126 in 1986), the W140 received a minor facelift. The clear turn signal indicator lenses on the front and rear were the most obvious changes. Headlamps were fitted with separate low and high beam reflectors. Following the mid-year facelift, the W140 coupe and sedan were each fitted with Electronic Stability Control. The W140 was to feature an air suspension as an option, but Mercedes was still perfecting the technology and chose to launch air suspension as an option in the next generation S-Class in late 1999.

Engines

In 1992, A new 6L 408 hp V12 engine joined the lineup for the first time with the 600SE, 600SEL and 600SEC. To distinguish these high-powered halo cars from their V8 and straight-6 counterparts, a "V12" badge was affixed to the C-pillar, a leather dashboard and suede headliner was available, and the grille was redesigned on all V12 models in 1997. In 1993, the 408 hp V12 motor was slightly detuned to 389 hp to comply with tighter emission control regulations in the United States and Europe. The V8 models were tuned down from 322 hp to 315 hp.