On February 13, 2020, the New York City Campaign Finance Board fined Monserrate over $26,000 for failing to obey various campaign finance regulations during his 2017 campaign for the city council of New York. The violations including exceeding the campaign spending limit. A spokesperson for Monserrate stated that Monserrate would pay the fines.

A grand jury indicted Monserrate on felony assault charges on March 23, 2009. Monserrate was charged with stabbing his girlfriend with a drinking glass in December 2008. Monserrate was cleared of the felony counts in October 2009 and convicted of misdemeanor assault. He was sentenced in December 2009 to three years probation. Mayor Michael Bloomberg (R) called Monserrate's actions "disgraceful, despicable, deplorable" but did not offer an opinion on his future, saying it was "up to the Senate." On February 9, 2010, the New York State Senate voted 53-8 to expel Monserrate. The vote came after approximately five hours of closed door negotiations by Senate Democrats. It was the first time since the 1920s that a member of the Legislature was forced out of office. Monserrate said that he planned to fight for his reinstatement in court, saying that only his constituents could vote him out. On February 19, Judge William Pauley of the United States District Court in Manhattan stated that the power of a legislative body to expel its members is "embedded in American Democracy," and denied Monserrate’s request to block his removal.