Melber worked for First Amendment lawyer Floyd Abrams at Cahill Gordon & Reindel from 2009 to 2013. He also began writing political columns for various news outlets like The Nation, The Atlantic, Reuters, and Politico. MSNBC took note and asked him to serve as a guest host. In April 2015, Melber was named their chief legal correspondent.

Melber was one of the many journalists to report that the firing of FBI Director James Comey could trigger an investigation into obstruction of justice. He reported on May 9, 2017, the day Comey was fired, that a former FBI official told him the firing raised the question of potential obstruction by President Trump. A probe was announced on May 18, 2017, which included an obstruction investigation of the Comey firing.

A few months into the Mueller probe, on August 29, 2017, Melber broke the story that a state investigator was exploring jurisdiction to charge potential defendants in the Mueller probe with state crimes, meaning a conviction would not be eligible for a presidential pardon. Politico followed up on the report the next day, and New York State prosecutors ultimately did file separate charges against Paul Manafort in March 2019, (after his trials in the Mueller probe).

Melber interviewed former Trump campaign manager Corey Lewandowski in Feb. 2019 about whether Trump asked him to interfere in the Mueller probe, and Melber later reported that Lewandowski’s response was false. In a congressional impeachment hearing in Sept. 2019, Lewandowski was questioned about his false answer.

On the night of the 2018 midterm elections, Melber broke a story live on MSNBC that Democrats on the Ways and Means Committee “intend to request President Trump’s tax returns.” In April 2019, United States House Committee on Ways and Means Chairman Richard Neal carried out that plan, formally requesting Trump’s tax returns from the IRS.

Melber has drawn attention for his interviewing and questioning skills. He was nominated for an Emmy Award for “Outstanding Live Interview” for interviewing four key witnesses in the Mueller probe at once. In November 2019, Columbia Journalism Review stated Melber is “a remarkably effective interviewer”, adding “his veins appear to contain ice water; he betrays no emotion at all” during intense exchanges. The New York Times columnist Peter Wehner, a former White House official in GOP administrations, said in February 2019, “Melber is an outstanding interviewer, among the best on television.”

In June 2020, conservative commentator Tiana Lowe wrote “Ari Melber on MSNBC” runs a “good straight news hour,” contrasting The Beat to other news programs. Mediaite wrote The Beat is a “thought-provoking” and “idiosyncratic show” that “avoids the singular focus on Trump’s misdeeds that consumes some other hosts”, adding Melber’s interviewing style uses “the facts of the story and logical reasoning [not] partisan cheap shots” for “fascinating” exchanges. Director Lee Daniels got emotional in a 2019 interview about his life and career with Melber, saying it was the only time he would ever “cry on television”.