Kermit Gosnell, 72, could face the death penalty after his conviction by a jury in a Philadelphia court, the district attorney's office said.
The jury returned guilty verdicts on the three babies, an acquittal in the alleged murder of a fourth, and a guilty verdict in the involuntary manslaughter of a woman on whom he was performing an abortion, DA's spokeswoman Tasha Jamerson said.
The jury, which had earlier Monday declared itself unable to decide on two counts before going back to try again, had been deliberating for 10 days.
Gosnell's crimes horrified Americans on either side of the intense national debate on the rights and wrongs of abortion.
Prosecutors said he performed late-term abortions in a state where the limit is set at 24 weeks and that when babies emerged still alive, he used scissors to sever their spines. However, he denied this, insisting that all babies were already dead as a result of the drug he was using in the abortion.
During one operation in the clinic a woman who'd come in for an abortion died from a heart attack after an overdose of anesthesia.
Gosnell's business, called the Women's Medical Society, was "decrepit and unsanitary," prosecutors said, staffed with unlicensed personnel who would "practice medicine on unsuspecting patients, unsupervised, and directed them to heavily drug patients in his absence."
The grand jury report indicting Gosnell described the outwardly professional looking brick building where he worked as a house of horrors.
"The clinic reeked of animal urine, courtesy of the cats that were allowed to roam (and defecate) freely. Furniture and blankets were stained with blood. Instruments were not properly sterilized. Disposable medical supplies were not disposed of; they were reused, over and over again," the report said.
"Scattered throughout, in cabinets, in the basement, in a freezer, in jars and bags and plastic jugs, were fetal remains. It was a baby charnel house."
The grand jury itself noted that the story would be "used" by both sides in the abortion debate.
Opponents have raised the case as evidence of abortion's fundamentally violent, criminal nature, while backers of abortion see the main lesson as the need to provide women -- especially the uneducated poor -- access to reliable, safe services.
John Boehner, the Republican speaker of the House of Representatives, called it "a sad and horrific case -- one that reminds us there is no one more defenseless, or more in need of our protection, than the unborn."