McNamee, testifying at Clemens' federal perjury trial, told jurors he "plunged the fluid" with a syringe into the hurler's buttocks while at the Toronto Blue Jays' training camp in Dunedin, Fla. -- the first of up to 10 times he did so that summer, The New York Times reported.
McNamee described the alleged first injection in great detail, saying he carried it out at the insistence of the ex-baseball great despite not knowing how to administer steroids, the newspaper said.
"I did what I was asked and shouldn't have," he told the jury.
Clemens, charged with lying to a U.S. congressional panel in 2008 when he denied using performance-enhancing drugs, has claimed the injections were not PEDs but the painkiller lidocaine and vitamin B-12.
McNamee also testified he was asked by Clemens to throw away a bag containing 20 to 30 ampules of testosterone at some point during the 1998 season.