The JS27 was a development of the previous year's JS25, with a lower fuel tank and revised aerodynamics to take advantage of the fuel limit which had been reduced from 220 to 195 litres. It was also lighter than its overweight predecessor, with the customer supply of Renault engines more effectively integrated into the overall package than before, although the specification of the engines was always slightly behind fellow-Renault users Lotus and was not developed through the course of the season.

Three JS27 chassis were built and ready to race for the first round of the championship in Brazil, and a further two were constructed during the course of the season: chassis 04 was introduced for the Belgian Grand Prix, whilst chassis 05 was ready in time for the race in Canada. Chassis 01 was written off due to the extensive front-end damage caused by Jacques Laffite's career-ending accident at the British Grand Prix.

The JS27 was initially driven by the French pairing of Ligier mainstay Laffite and René Arnoux. Both drivers were renowned as being fast and experienced, but their age and setup skills were cast into doubt before the start of the season. Arnoux, in particular, had missed all but the first race of the 1985 season after being mysteriously sacked after the Brazilian Grand Prix by Ferrari (neither Ferrari nor Arnoux have ever gone public with the reason for his sacking), and at 42 years old, Laffite was the oldest driver on the grid.

The JS27 was instantly competitive, however, with numerous points finishes in the first half of the season, including two podium finishes from Laffite. At Detroit, both drivers were particularly fast, Laffite leading the race and eventually finishing second, whilst Arnoux looked on course for second place until he crashed. By the time of the British Grand Prix, Ligier were fourth in the Constructors' Championship: behind Williams, McLaren and Lotus, but ahead of Ferrari.

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