San Francisco’s top transit boss is stepping down, the latest fallout from a 10-hour subway meltdown that choked the city on Friday.

Ed Reiskin, who has steered San Francisco Municipal Transportation Agency for eight years, made no mention of the incident in a contrite note he sent to colleagues on Monday. But to most observers, the connection was clear.

“The employment agreement I have with the SFMTA Board of Directors ends in August, and it’s become clear that this is the right time for a change,” Reiskin wrote. He said he will resign this summer, shortly before his contract expires.

His announcement coincided with a scorching letter from Mayor London Breed — the latest in a series of missives to the city’s main transit board — calling for a national search to find a new director.

The failure of an overhead wire that crippled the downtown commute on Friday appeared to be the breaking point for Breed, who was already scrutinizing the agency.

“While unplanned outages and service disruptions come with a system as complex as ours, it is our proactive work to prevent these outages and disruptions from occurring and responding to them head-first when they do that defines strong leadership,” Breed wrote.

“San Franciscans deserve robust contingency plans that are ready to execute, strong communication plans that are ready to deploy, and a visibly engaged management team that can steer the system through turbulence,” she continued. “Yet recent incidents at the SFMTA have shown that existing leadership is not up to the task.”

Reiskin took the job in 2011, a novice to the transportation world with plenty of experience running other embattled departments — he’d headed the city’s 311 call center for complaints and the Department of Public Works.

An avid cyclist and public transit rider known for wearing secondhand suits, Reiskin cut an unassuming figure. He endeared himself to many San Francisco residents while attracting criticisms from others — including Breed — who wanted a tougher, more ambitious personality heading the department.

On Monday, Breed described what she is looking for to reporters at City Hall.

“The goal is to find a visionary leader who is prepared to really run this department and lead ... in a way that will restore the trust in our transit system in San Francisco,” she said.

Some expressed skepticism that anyone who is truly qualified would want Reiskin’s $342,483 job managing the streets, parking and public transit of an evolving city. It’s a role at the center of a political maelstrom, fighting for street improvements that require the removal of parking spaces; rallying for infrastructure money in the city budget; and balancing the interests of merchants, bike commuters, motorists, e-scooter companies, pedestrians and politicians who all spar with one another.

“Every politician says, ‘I want a visionary, I want an independent leader,’ — but my experience is, that’s not actually what they want,” said Tom Radulovich, a former BART board director and executive director of the nonprofit group Livable City.

“They want a leader who won’t rock the boat,” Radulovich added. “‘No bad news’ tends to be the best way to run a transportation department.”

Still, Reiskin’s low-key style only worked for so long.

He came under increasing pressure as Muni’s challenges mounted. Last summer the agency poached buses and drivers from popular lines to run shuttles around the Twin Peaks Tunnel, which had closed for a two-month retrofit. The strategy exacerbated a driver shortage that had dogged Muni for years, slowing down service throughout the system. Worse yet, officials failed to explain the patchwork plan to riders, who were left stranded and confused at bus stops.

Those problems escalated over the ensuing months, as lawsuits and employee complaints exposed a culture of bullying and intimidation within the agency. The former director of the Muni bus and subway line abruptly retired in October amid accusations of sexual harassment from an employee, just as the SFMTA was trying to roll out a new fleet of rail cars, which are racking up rider complaints and concerns about safety.

What was supposed to be a period of expansion and innovation instead became a downward slide. Demand for bus and rail service increased in booming San Francisco, posing challenges for an agency hobbling along on old equipment. Managers at SFMTA struggled to meet basic benchmarks, like the goal that acting transit director Julie Kirschbaum set to reduce major, incapacitating subway delays to four per month. Riders saw five delays of 20 minutes or longer in March.

In her letter, Breed acknowledged that San Francisco had failed to adequately invest in its transit system for decades, and the strain is starting to show. Population and job growth means that more people rely on Muni, and city politicians have less patience for snafus.

Yet she also criticized Reiskin and other SFMTA leaders for appearing “paralyzed” when issues piled up. The agency hesitated to crack down on alleged harassment until Breed hired an ombudsman to intervene. Managers hedged when questions arose about door sensors on the new Siemens rail cars after a woman’s hand got stuck earlier this month, sending her tumbling onto the tracks.

And it wasn’t until 9:30 p.m. Friday that the agency explained the daylong collapse in its subway system, which elicited questions about the maintenance of its overhead wires.

Meanwhile, the shortage of bus and train operators persists. The operators organized a labor action last week, declining to work on their days off as they negotiate a three-year contract. It caused delays on more than a dozen bus lines.

Calling for new leadership is a political risk for Breed, who has effectively taken ownership of the agency. Yet some of her City Hall peers say the mayor has no choice.

“I don’t think it’s surprising,” said Supervisor Rafael Mandelman, noting how much the events of the past few months have infuriated residents and workers. San Franciscans “think the mayor is responsible for everything that goes wrong in the city,” he said.

Malcolm Heinicke, who chairs the SFMTA’s Board of Directors, applauded Reiskin on Monday for a tenure marked by “dedication and persistence.”

“The way Ed is departing really speaks to his character,” Heinicke said. “He’s not complaining about the situation. He’s not saying he was mistreated — none of that. He’s saying, ‘It’s time for me to leave at the end of my contract.’ But he’s going to carry through and work hard to the end.”

Heinicke put Muni under a microscope after taking over as chair this year, demanding monthly performance reports for the light rail and buses. He said that in previous years the board had been fixated on big-picture plans rather than day-to-day service, which is what stands out most to commuters.

“The challenge is that we have an aging system that wasn’t designed perfectly,” Heinicke said. “But we’ve all been on the system when it runs properly, and we all know how great that can be.”

Rachel Swan is a San Francisco Chronicle staff writer. Email: rswan@sfchronicle.com

Twitter: @rachelswan