TOKYO — Osamu Masuko, the long-serving, soft-spoken chairman of Mitsubishi Motors Corp. who hammered out his company’s partnership with Nissan Motor Co., leaves behind a stronger carmaker but one that is still a work in progress after several fitful revival attempts.

Masuko died of heart failure Aug. 27, three weeks after stepping down over health concerns. He was 71. At the time of his retirement, when he took a “special adviser” role, Masuko was one of Japan’s oldest and longest-serving automotive chiefs. He led Mitsubishi for 15 years.

Masuko became president of Mitsubishi Motors in 2005 and cycled through various positions at the top, including the CEO and chairman posts, during a pivotal period at the small Japanese player. Masuko hatched numerous plans to reboot the brand’s flagging fortunes, oversaw its foray into electrified vehicles and helped pen its 2016 partnership with Nissan.

With Mitsubishi then part of the Renault-Nissan alliance, Masuko exerted a stabilizing influence over the company when the alliance was nearly torn apart by the 2018 arrest of Carlos Ghosn, who was ejected from the group after positioning himself as chairman of all three companies.

“Masuko-san devoted himself to leading and growing the company for 15 years,” Nissan CEO Makoto Uchida said in a statement. “He played an instrumental role in the negotiations with Nissan that led to Mitsubishi Motors joining the alliance in 2016. His wisdom and foresight will remain as an inspiration to the automotive industry, and we will always honor his memory.”

In his last year at the helm, Masuko was preoccupied with soothing the turmoil at Renault-Nissan-Mitsubishi and trying to reposition Mitsubishi for a more profitable future.

Masuko coined a new strategy called “Small But Beautiful,” refocusing Mitsubishi on the Southeast Asian market where it is strong and scaling back in other markets where it is weak.

But the turnaround was broadsided by the COVID-19 pandemic. Mitsubishi now expects to post a net loss for the fiscal year ending March 31, 2021, for the second consecutive year. The forecast calls for the red ink to balloon nearly 14-fold from the loss reported the previous year.

Masuko ascended to the top of Mitsubishi Motors even though he transferred into the company late in his career, a rarity in Japan, where company lifers typically climb to the top of the ladder.

He was sent by Mitsubishi Group’s trading firm Mitsubishi Corp., then one of its top shareholders, to help revive the carmaker after its brief marriage and messy divorce with DaimlerChrysler.

He stepped in just before the Great Recession throttled the global auto industry.

Mitsubishi’s U.S. sales fell by more than half to 53,986 vehicles in 2009, from 128,993 vehicles two years earlier. Its volume hovered below 100,000 a year until 2017 and grew to 121,046 in 2019. But the brand’s U.S. sales still languished far below the record 345,111 booked in 2002.

Masuko pushed ahead with rebranding Mitsubishi as a purveyor of electrified vehicles, crossovers and SUVs.

While other auto chiefs would be chauffeured around town in luxury limousines, Masuko would shuttle around Tokyo in a red-and-white i-MiEV. The four-passenger minicar, launched in 2010, was Mitsubishi’s first mass-market EV. And Mitsubishi tried to carve out a niche spanning the segments with its Outlander PHEV plug-in hybrid crossover.

But fragile finances meant Mitsubishi was usually strapped to invest deeply in new technologies for electrification and autonomous driving. And pinched for cash, the company dropped some of its long-running passenger cars, including the Eclipse sporty car and Lancer and Galant sedans.

Masuko, then 65, seemed poised to ease himself into retirement in 2014, when he handed the presidency to Tetsuro Aikawa, a veteran engineer noted for leading development of such Japan-market minicars as Mitsubishi’s eK Wagon and the egg-shaped i.

Masuko took the then-vacant CEO position in addition to remaining chairman.

As part of the revival plan under Masuko and Aikawa, Mitsubishi decided to shutter its only North American assembly plant, the underutilized factory in Normal, Ill., it opened in 1988 as a joint venture with then-partner Chrysler.

Mitsubishi also dangled plans for various new EVs and plug-ins, none of which materialized. But any rebound was derailed in 2016, when Mitsubishi admitted it had been overstating fuel-economy ratings on four minicar models sold in Japan by Mitsubishi and Nissan.

The bogus fuel-economy results were discovered when Nissan noticed discrepancies and called out Mitsubishi. Mitsubishi’s sales tanked, and so did its share price.

Mitsubishi’s plunging stock created a window for Nissan, then run by Ghosn, to swoop in under the pretense of a white knight and take a controlling 34 percent in Mitsubishi.

Aikawa resigned to take responsibility for the scandal, and Nissan sealed the new partnership in late 2016. Ghosn installed himself as Mitsubishi chairman, and Masuko stayed on as CEO for continuity.

When Ghosn was arrested in 2018, fired as chairman and then dismissed from the Mitsubishi board in early 2019, Masuko was the last man standing. Masuko began paving the way for a new generation of leadership with the appointment of Takao Kato as CEO in June 2019.

Kato was delegated temporary chairman when Masuko stepped down Aug. 7.

After Ghosn’s arrest, Masuko expressed shock and disappointment in his former boss. But even as the alliance struggled to find its footing, Masuko still saw safety in numbers.

“In this time of great change, it is not realistic to respond to all these changes,” Masuko said last year of automakers that would go it alone. “There are a number of benefits we have gained from the alliance. … For advanced technologies, it is beneficial for us to tackle them through the three-party alliance. I think the alliance will generate benefits for us.”