Joe Lhota returned as MTA chairman last summer with a big mandate: overseeing a sprawling network of bus, subway and commuter rail lines.

"We've got to take this system and get it out of the late 19th century and get it into the 21st century as quickly as we possibly can," Lhota said in July 2017.

But it wasn't his only job. Lhota remained as chief of staff at NYU Langone Health, which pays $1.6 million a year.

And, it turns out, Lhota also has been sitting on the board of directors of the Madison Square Garden Company, an affiliation not widely known until a report Tuesday by the website Politico New York.

Some watchdog groups immediately criticized Lhota's MSG role, pointing out that the Garden sits atop Penn Station, the MTA's biggest transit hub. 

"The MTA has major, major real estate dealings with MSG and the properties around it. And this is direct, direct conflict of interest with Lhota's job," said John Kaehny, executive director of Reinvent Albany.

Kaehny said Reinvent Albany will file a complaint with the state's Authorities Budget Office. 

"We're going to highlight the fact that Lhota joined the board of Madison Square Garden without telling the MTA board, which we believe violates the Public Officers Law," Kaehny said.

The MTA says Lhota received a letter from the state's Joint Commission on Public Ethics allowing him to serve on the MSG board.

A spokesman added, "If there is anything that involves Madison Square Garden and Penn Station, Lhota will appropriately recuse himself from any matter where there could be even the appearance of a conflict."

Lhota returned to the MTA in June for a second run as chairman, a post he left for a failed mayoral bid in 2013. 

As with the MTA, Lhota has an extensive past with the Garden, having spent nearly a decade working for the family that owns the Knicks, the Rangers and the arena.

Nicole Gelinas of the Manhattan Institute said state lawmakers who approved Lhota's nomination should have known of his ties to a company doing business with the MTA. 

"It's the state legislature's job to oversee the governor's decision here and say, 'Wait a minute, this doesn't seem to make a lot of sense,'" Gelinas said.

The Authorities Budget Office promised to make public the findings of any investigation.