Adenhart starred at Williamsport High School before becoming a major leaguer. Those wearing baseball clothing at the service did so at the request of the family.
The 22-year-old pitcher was killed in a car accident in California on April 9, hours after pitching six scoreless innings against the Boston Red Sox. He was buried Thursday in a private ceremony.
His friends Courtney Stewart, 20, and Henry Pearson, 25, also died in the crash.
Friday's memorial service was held in the school gym. Outside, a large circle containing Adenhart's name and his No. 34 with the Angels was displayed. Inside the gym, a video depicted him at various stages of his life, celebrating his outgoing personality and prowess on the mound.
Several photo montages were set up between floral arrangements sent by the Angels, Major League Baseball, the Double-A team he played for in Arkansas, Angels teammate Torii Hunter and the Boston Red Sox.
The eulogy was to be given by the Rev. Josh Carter, a high school teammate before Adenhart transferred to Williamsport.
Adenhart's stepfather, Duane Gigeous, said the family would not speak to reporters before or after the service.
Andrew Thomas Gallo was charged with three counts of murder after police said he ran a red light and hit the car carrying Adenhart and his friends.