He spoke eight times with Robert S. Mueller III’s investigators, appeared before three congressional committees and helped federal prosecutors with at least two criminal inquiries into President Trump’s inner circle. But on Monday, perhaps the most important number in Michael D. Cohen’s life will be 1,095: the number of days in a three-year prison term.

Mr. Cohen, the president’s former fixer and current antagonist, reported to the federal prison in Otisville, N.Y., on Monday to begin serving his sentence for campaign finance violations, tax evasion and other crimes. He pleaded guilty last year to arranging a hush money scheme in the final weeks of the 2016 presidential campaign, involving two women who said they had had affairs with Mr. Trump.

Before leaving Manhattan Monday morning, Mr. Cohen spoke briefly with reporters outside his residence at Trump Park Avenue.

“I hope that when I rejoin my family and friends, that the country will be in a place without xenophobia, injustice and lies at the helm of our country,” he said before ducking into a black SUV for the two-hour drive to Otisville. “There still remains much to be told, and I look forward to the day that I can share the truth.”