Deputy Washington County State's Attorney Charles Strong said David Knapp, 19, went into the American Trust Bank at 14 W. Potomac St. at 12:20 p.m. wearing an orange ski mask and demanded money.
As he left with $7,288, he was spotted by a bank customer who saw him remove his mask and throw something into a car driven by another man, Strong said.
That customer followed the suspect through Williamsport in his car, copying down the make, model and tag number of the getaway vehicle.
Based on the description, deputies arrested the driver a short time later.
Meanwhile, David Knapp had run into an alley about a half-block from the bank. Washington County Sheriff's Deputy Kenneth Cain and Maryland State Police Trooper 1st Class D.A. Harper and Sgt. Steve McCarty arrested David Knapp near the Williamsport American Legion, authorities said.
Sheriff's deputies later found a jacket, mask, backpack and a ratchet in the alley behind the bank.
Sheriff's Department Sgt. Randy Wilkinson said authorities initially thought the robber had a gun but none was found.
Scheduled for a pre-trial court hearing Thursday, David Knapp instead pleaded guilty to one count of robbery.
In exchange for that plea, armed robbery, theft, first- and second-degree assault and conspiracy to commit robbery charges were dropped.
"Even though a weapon wasn't used, others were placed in fear," said Washington County Circuit Judge Frederick C. Wright III as he levied the sentence.
David Knapp told Wright he knew he had done wrong that day.
"But I want you to know I wrote letters to both the bank tellers and apologized," David Knapp said.
Both Inwood, W.Va., residents, the Knapps have been held in the Washington County Detention Center since the robbery.