In a move sure to fray his relations with law enforcement, President Obama on Thursday appointed Debo Adegbile, a former attorney for convicted cop killer Mumia Abu-Jamal, to a six-year post on the U.S. Commission on Civil Rights.

Mr. Adegbile worked at the NAACP Legal Defense and Educational Fund when he represented Abu-Jamal in the appeal of his conviction and death sentence for the notorious 1981 shooting death of Philadelphia police officer Daniel Faulkner. Abu-Jamal’s sentence was reduced to life in prison.

The case prompted the Senate to reject Mr. Adegbile’s nomination in 2014 when Mr. Obama appointed him to lead the Justice Department’s office on civil rights. Some Democrats joined Republicans in voting down the selection at that time.

Liberals praised the latest nomination. The Center for American Progress cited Mr. Adegbile’s “work on employment, housing discrimination, criminal justice and voting rights.”

Mr. Obama also appointed Catherine Lhamon, assistant secretary for civil rights at the Education Department, to a six-year seat on the commission. She drew the ire of Republican lawmakers in 2011 for issuing a “dear colleague” letter to colleges advising them to lower their standards for judging students accused of sexual harassment of bullying, touching off a dispute in Congress over the legal authority of the letter.

The Center for American Progress praised both appointees as “individuals with a long and proven history of commitment to the advancement civil rights.”

The Senate’s rejection of Mr. Adegbile on a 52-47 vote in 2014 marked the first time that the Senate rejected one of Mr. Obama’s nominees since Democrats changed filibuster rules to require only a simple majority for presidential nominations, an easier threshold that the Adegbile pick still failed.

Mr. Adegbile drew opposition from national law enforcement organizations as well as a bipartisan group of senators for his involvement with Abu-Jamal, who became a cause celebre from his prison cell.

After the nomination was defeated, Mr. Obama called the vote a “travesty based on wildly unfair character attacks against a good and qualified public servant.”

“Those who voted against his nomination denied the American people an outstanding public servant,” Mr. Obama said.

At the time, Sen. Patrick J. Toomey, Pennsylvania Republican, said Mr. Adegbile had “decided to join a political cause. … And in my view, by doing so, he demonstrated his own contempt for — and frankly a willingness to undermine — the criminal justice system of the United States.”

“I do not believe that Mr. Adegbile’s nomination is consistent with justice for the family of officer Danny Faulkner or for anyone else that cares about the law enforcement community,” Mr. Toomey said at the time.

The eight-member civil rights commission consists of four members appointed by the president and four appointed by Congress. The six-year appointments are not subject to Senate confirmation.

The commission is an independent, bipartisan, fact-finding federal agency with a mission to “inform the development of national civil rights policy and enhance enforcement of federal civil rights laws,” according to its website.