Father Heinrichs served in various positions in the New York and New Jersey area including pastor at Holy Angels parish in Singac (Little Falls), New Jersey, at St. Stephen’s in Croghan, New York, and at St. Bonaventure’s between 1891 and 1907. When he was pastor at Paterson,  smallpox broke out and he was known to spend many hours at a nearby "pest house" tending to the sick and the dying. In September, 1907, the Provincial Chapter appointed him pastor of St. Elizabeth’s parish in Denver, Colorado where he arrived on September 23. He had but 5 months to live. He had received permission to leave for Germany to visit his family who had not seen him for over twenty years. But he had a class of children preparing for their first Holy Communion and he was determined to give them First Communion on June 7, 1908.

A week before his death, Father Leo told the Young Ladies’ Sodality "If I had my choice of a place where I would die, I would choose to die at the feet of the Blessed Virgin."

Among those at Mass that morning was fifty year old Giuseppe Alia, who had recently immigrated from Italy. Alia arrived before Mass and seated himself in the third row, in front of the pulpit.

During Communion, Alia knelt at the Communion Rail and received the Host. Then, however, he spat it into his hand and flung it at Father Leo’s face. The Host fell to the floor as Alia drew his gun aiming at Father Heinrich's heart. As an altar boy screamed the man opened fire. The dying priest exclaimed, "My God, my God!," before falling to the floor. Before he died, he placed the ciborium on the step of Our Lady’s altar, and managed to place two fallen Hosts back into the ciborium before strength left him and with his last bit of strength he pointed to the spilled Hosts that he was now too weak to pick up. Rose Fisher, an eyewitness, reported that Father Leo died smiling, at the foot of the Blessed Virgin's altar just as he had always wanted. Father Wulstan Workman, who had switched with Father Leo for the later Mass, administered the Last Rites. Father Wulstan told the Denver Post, "I would have been killed and he would be alive now. There is one way to solve the affair that I can see, and that is that God chose the better man."

Father Leo's body was transported to New Jersey for burial in a Franciscan cemetery. Saint Elizabeth of Hungary still stands, and now serves both the Roman Catholic church and Denver's Russian Catholic community.

Guiseppe Alia attempted to flee the Church, but E.J. Quigley, a conductor for the Denver & Rio Grande Railroad, caught him. Then, Patrolman Daniel Cronin, an off-duty Denver police officer placed him under arrest and had him jailed.

At the police station, Alia boasted of his Anarchist beliefs, saying,

Alia was tried, convicted, and sentenced to death by hanging within weeks of the shooting. Shortly before the execution, a Franciscan priest from St. Elizabeth’s visited Alia in prison. Infuriated, Alia cursed and swore at him. Alia never expressed any remorse, and, despite the pleas of the friars at St. Elizabeth’s, he was hanged at the Colorado State Penitentiary in Canon City. Alia’s last words, reportedly, were "Death to the priests!"

The coroner discovered that Father Leo’s upper arms and waist were wrapped in leather straps. Each strap was studded with rows of pointed iron hooks, which pierced the skin. Around the priest’s waist the skin was calloused and scarred, but showed no sign of infection. Father Leo secretly practiced this extreme form of mortification, perhaps to help him master his quick temper. None of his confreres had any idea of his self-inflicted penances. When the friars entered Father Leo’s room after his death, they found that he slept on a wooden door."

In Germany Erkelenz has named a street after Fr. Leo Heinrichs. The process of beatification has been opened in Rome since 1938 and his tomb in Totowa, Holy Sepulchre Roman Catholic Cemetery, remains a place of veneration.