The W09 was designed under James Allison and maintained the identical 3,726-millimetre wheelbase of its predecessor, while increasing the car's rake angle from 0.9 degrees to 1.2 degrees — still shallower than Ferrari's 1.5 degrees or Red Bull's 1.9 degrees. A new rear suspension geometry used a long titanium extension from the wheel hub to the upper wishbone, creating clearer airflow between sidepods and diffuser.
Development through the season was significant. A spoon-shaped rear wing section with vortex-generating serrations was trialled in Azerbaijan. Revised front suspension and steering assemblies addressed the car's early weakness with tyre temperature management. Major aerodynamic changes at the Austrian Grand Prix revised the radiator inlets and bargeboard layout. A concave rear brake drum introduced at Singapore, combined with a ribbed rear wheel rim from Belgium, was designed to manage rear tyre temperatures. However, Mercedes was later suspected of using blown holes in the rear wheels for aerodynamic benefit — a practice the team denied — and closed those holes before the final races, coinciding with a dip in performance.
The Mercedes-AMG F1 M09 power unit underwent complete redesign for 2018. It introduced a qualifying "party mode" delivering a performance boost over a single lap. Three engine specifications were deployed during the season: the opening specification from Australia through Canada, a Phase 2.1 unit from France, and a third specification from Belgium. For the first time since 2014, the Mercedes power unit trailed Ferrari's 062 EVO in straight-line performance.
The early season favoured Ferrari. Vettel won in Australia after a software glitch cost Hamilton a likely victory, again in Bahrain, and in Canada, where Hamilton could manage only fifth. Through four races Ferrari led the Constructors' Championship. The turning point came at the Spanish Grand Prix, where Hamilton and Bottas locked out the front row and delivered Mercedes' first 1-2 finish of the year, resolving the tyre management issues that had troubled them earlier.
Hamilton's championship lead grew inconsistently. A tactical victory in Germany came after Vettel retired in wet conditions from which Hamilton had started fourteenth after power failure in qualifying. Hamilton's win in Italy, where he surpassed Michael Schumacher's all-time pole position record with his sixty-ninth career pole, was another pivotal moment; he took the championship lead after Vettel finished fourth.
Mercedes controlled the final phase of the season. Hamilton and Bottas recorded back-to-back 1-2 finishes in Russia and Japan. The Constructors' Championship was clinched at Brazil; Hamilton wrapped up the Drivers' title at Mexico with a fourth-place finish. The team completed 98.6 percent of all race laps through the year, recording only one retirement across both cars — Bottas's retirement in Austria.
The 2018 season presented Mercedes with its most serious title challenge since 2010. Ferrari's SF71H showed superior pace at multiple circuits across the first two-thirds of the year, and the performance gap between the two constructors fluctuated across race weekends. Vettel led Hamilton in the Drivers' Championship for extended periods, and Ferrari held the Constructors' lead after four of the first five rounds. Mercedes ultimately prevailed through stronger reliability, better tyre management after the Spanish upgrade, and Hamilton's capacity to deliver decisive performances when the championship required them.
The W09 demonstrated that Mercedes could retain their competitive edge even when a rival presented a genuinely comparable or faster car at multiple venues. Hamilton's fifth title — equalling Juan Manuel Fangio's total — left him one behind Michael Schumacher's record of seven. The 2018 season also marked the moment when engine-mode transparency became a political issue in Formula One, with the "party mode" concept setting up a formal ban by the FIA in 2020.