The incident comes as Mamdani is already facing criticism over his decision to attend New York City's 25th anniversary 9/11 remembrance ceremony. Some victims families and former city officials have asked him to stay away.
Mamdani recently signed executive orders declaring September 11 a citywide day of remembrance and service and taking other steps related to emergency preparedness.
After signing the orders, Mamdani handed the ceremonial pen to NYPD Commissioner Jessica Tisch and asked her to pass it to Kassem, his chief counsel. The moment has now become a fresh point of criticism against the mayor.
Kassem previously represented Ahmed al-Darbi, a Guantanamo detainee who pleaded guilty to war-crime charges linked to al Qaeda maritime plots. Al-Darbi was the brother-in-law of Khalid al-Mihdhar, one of the hijackers aboard American Airlines Flight 77, which crashed into the Pentagon on September 11, 2001.
City Hall has said Kassem was present because he and his team had worked on drafting the executive orders as part of his official duties.
Criticism over Mamdani's 9/11 appearance
The pen incident has added to the criticism over Mamdani's plan to attend the 25th anniversary ceremony at Ground Zero.
A petition backed by relatives of 9/11 victims has collected nearly 100,000 signatures calling on him not to attend, according to ABC News. Former officials, including former FDNY Commissioner Tom Von Essen, have also criticised his planned presence.
Mamdani, however, has said he will attend to honour those who died and the first responders.
Signal group with more than 200 influencers
Another recent controversy involves the Mamdani administration's use of Signal to communicate with more than 200 social media influencers.
Reports said the group was used to share City Hall updates, talking points and video material with creators. The use of an app with disappearing messages has raised questions about government record-keeping and transparency.
Mamdani said his administration would follow all legal requirements for preserving city records and would take corrective action if any rules were broken.
City-run grocery stores face a new lawsuit
Mamdani's plan to open five city-run supermarkets has also come under fresh legal pressure.
A group representing private supermarkets filed a federal lawsuit on September 9, arguing that the city-backed stores could use public money and subsidies to sell goods at prices private stores cannot match. The plan includes a proposed 30 percent discount on basic goods such as meat, dairy and produce.
The administration has defended the plan and says it is aimed at making groceries more affordable for New Yorkers.
AI ban in schools
Mamdani also faced criticism after New York City announced a one-year ban on the use of AI by students in public elementary and middle schools.
The policy affects around 600,000 students through eighth grade and stops the use of about 40 educational AI tools in classrooms. High school students will still be allowed to use AI in some cases, while teachers can use it for tasks such as lesson planning.
Mamdani said the move was meant to ensure that children continue to develop basic skills and human connections without depending too much on AI.
Netanyahu arrest issue returns
Mamdani's earlier comments about Israeli Prime Minister Benjamin Netanyahu have also returned to the spotlight as Netanyahu prepares to visit New York for the UN General Assembly.
Mamdani had called for Netanyahu's arrest over the International Criminal Court warrant but later acknowledged that the New York City mayor does not have the power to arrest him. He has instead called on the US federal government to enforce the warrant.
Who is Mamdani and what he thinks of India?
Zohran Mamdani is the Democratic mayor of New York City and the first Muslim and Indian-American to hold the post. Born in Uganda to Indian-origin parents, Mamdani has deep family roots in Gujarat and has spoken openly about his Indian identity.
However, his views on India have made him controversial among sections of Indians, particularly supporters of Prime Minister Narendra Modi and the BJP. Mamdani has strongly criticised Modi over the 2002 Gujarat riots, calling him a “war criminal” and accusing him of responsibility for the violence against Muslims.
He has also criticised what he describes as the BJP and Hindu nationalist movement’s “exclusionary” vision of India. Most recently, he opposed an event in New York featuring RSS Sarsanghchalak Mohan Bhagwat, saying the India he was taught about was a “pluralistic society” and a “secular Republic” where everyone belongs.