Martin’s cousin and accomplice in the crime, Roger Clark III, pleaded guilty a year ago to first-degree murder in Diener’s killing.
Martin’s lawyers repeatedly said through the trial that Martin was innocent.
Diener’s sister, Patti Diener Lough, said after the verdict that it was a “sad day.”
“We are certainly not celebrating,” Lough said. “This is going to impact the lives of two young men and that is heartbreaking. But there are consequences to taking a life.”
The cousins lived in the Northwest Washington home of Clark’s father, a District lawyer. Martin and Clark were out the night of Dec. 29 drinking and gambling with friends in D.C. when they decided to drive to Arlington to rob someone to recoup their gambling losses, according to prosecutor Molly Newton.
They came across Diener, 57, as he walked in the dark to his job at Arlington Sport and Health Club, a mile away from his Lyon Village home.
Clark jumped him while Martin stayed in the car, Newton said. But Diener, a strong man in top physical condition, fought back so hard that Martin jumped out of the car to intervene and fatally stabbed Diener, Newton said.
Clark then went through Diener’s pockets, leaving his DNA at the scene. Martin grabbed Diener’s gym bag, which contained $50, Newton said.
“Carl Diener died for $50,” Newton told the jury.
Clark is awaiting sentencing. Prosecutors offered Clark a reduced sentence if he testified against his cousin, which he did last week.
Diener was a neighborhood fixture, a gregarious man who exercised regularly and enjoyed a circle of friends at the gym. Diener did not own a car or cellphone and walked to work each day before sunrise.
He had worked at the gym for about 10 years, taking an early shift so he could keep his government job, from which he retired about a year before he was killed. He was a former D.C. health inspector and also worked for the General Services Administration.