A former doctor who treated patients at the D.C. jail was sentenced to five years in prison Friday for sexually assaulting an inmate during an exam.

D.C. Superior Court Judge Herbert B. Dixon Jr. also required Lewis Jackson, 37, to register as a sex offender and to undergo a mental evaluation and treatment in prison.

Earlier this year, a judge in Atlanta sentenced Jackson to 25 months in prison for a separate case in which he was charged with sexually assaulting three inmates during examinations in 2011 at a Georgia jail.

On Friday, Assistant U.S. Attorney Peter V. Taylor called Jackson a “sexual predator,” saying that he targeted inmates because he thought authorities would not believe them if they reported the abuse.

“He used his position to prey upon the most vulnerable population — inmates — and sexually abuse these men,” Taylor said.

In January, Jackson pleaded guilty in the District to one count of second-degree sexual abuse. At Friday’s hearing, Dixon also ordered that Jackson be supervised for 10 years after his release from prison.

From 2007 through 2009, Jackson was employed by United­Healthcare in the District, which provides medical services to the jail’s inmates. In 2008, prosecutors said, Jackson assaulted the inmate. When detectives confronted Jackson about the incident, he denied it and moved to Atlanta.

In Atlanta, Jackson was accused of assaulting three inmates. One cooperated in an investigation and wore a wire, recording Jackson and providing evidence needed for an arrest, authorities said.

Jackson’s attorney, Gregory G. Marshall, said that Jackson has surrendered his medical licenses and that his actions were a result of mental illness that was not diagnosed or treated until after his arrest. He said Jackson has bipolar disorder.

“Had he been treated for his mental illness, none of this would have happened,” Marshall said.

Jackson’s parents, sister and wife sat in the courtroom as he apologized to them and his victim. At times, he was in tears.

“I worked hard in school, in my community, tried to give back. Then after all that, I made a terrible mistake,” he said, reading from a statement. “My conduct was inexcusable. I let my parents, my wife, my church and myself down. And I let down my community and my profession.