Damian Bradford apologized in federal court to the doctor's family, saying he regretted killing Dr. Gulam Moonda, who had never even spoken to him.
"I can't express how sorry I am for the action that occurred. I made the decision to do what I did," Bradford said.
Prosecutors agreed to the plea in exchange for Bradford's testimony against Donna Moonda, who was convicted last week of murder-for-hire in her husband's slaying.
Bradford testified that he followed the couple from their home in Hermitage, Pa., and pulled behind the Moondas' Jaguar as they stopped along the Ohio Turnpike. He said he ran to the passenger side of the car and shot the doctor.
Prosecutors argued Moonda, 48, grew tired of her marriage and offered Bradford, 25, a share of her husband's estate if he killed the 69-year-old doctor. Her defense argued Bradford acted alone.
The doctor had more than $3 million, plus life insurance policies totaling $676,000.
Bradford, of Monaca, Pa., had faced 30 years to life in prison on charges of interstate stalking and using a firearm to commit a violent crime.
CBS affiliate WOIO reports prosecutors asked for a shorter sentence because his testimony helped convict Moonda.
Donna Moonda's sentencing hearing is scheduled July 16; she could receive the death penalty or life in prison without parole.
Her attorney, Lawrence Whitney, said the fact that Bradford, the triggerman, received just 17 1/2 years in prison would be an important factor in asking the jury to spare Donna Moonda's life.
A letter from the doctor's nephew, Faroq Moonda, asked the judge to impose a sentence against Bradford worthy of his crime. The letter described Gulam Moonda's rise from poverty in India to prosperous life in America as a respected urologist who donated money to schools and charities and sent money to relatives in India.
Prosecutors read the family letter, then asked for the agreed-upon sentence, noting that Bradford's cooperation helped convict the doctor's wife.
Bradford, a former drug dealer and abuser who met Donna Moonda in drug rehab, said he thinks daily about taking the doctor's life.
"If I would have gotten away with it or not, I don't think I would have been able to live with this feeling in my gut," he said.