After months of political turmoil, the Chicago Board of Education voted unanimously to fire the chief executive of the city’s public school system on Friday night. The move to oust the executive, Pedro Martinez, represented a show of force for the Chicago Teachers Union and for Mayor Brandon Johnson, a close ally and former employee of the union.

There had been a crescendo of chaos at the district all fall that included Mr. Martinez resisting a proposal to take out a high-interest loan to help cover a budget gap, the entire School Board resigning, and the president of Mr. Johnson’s handpicked replacement board quitting after elected officials criticized his old social media posts as antisemitic and misogynistic.

All that played out amid a drawn-out fight over Chicago’s budget, an ongoing contract negotiation with the powerful teachers’ union and painful questions about whether a post-pandemic surge in school staffing and services would be financially sustainable.

After months of behind-the-scenes maneuvering to oust Mr. Martinez, the discord reached its climax at an evening board meeting on Friday, just weeks before Chicago shifts to a new system in which a portion of the school board is elected. Mr. Martinez has, through his lawyer, threatened to sue if he was fired or the board sought to limit his power.

That lawyer, William J. Quinlan, accused the mayor and the union of campaigning to “improperly and unlawfully terminate Mr. Martinez based on wholly pretextual reasons.” Neither the mayor’s office nor the union immediately responded to a request for comment on Friday.

The dispute over Mr. Martinez’s job status centered on some of the most vexing post-pandemic challenges that big city school systems are wrestling with: under-enrollment, budget crises and debates over just how powerful public employee unions should be.