[4][3] Hartmayer was ordained to the priesthood at the Cathedral of the Immaculate Conception in Albany, New York, for the Conventual Franciscans by Bishop Howard J. Hubbard on May 5, 1979.

[1] Following a three-month sabbatical at St. Patrick Seminary in Menlo Park, California, Hartmayer briefly served as an instructor at John Carroll Catholic High School in Fort Pierce, Florida, in 1995.

The diocese reached a settlement in 2016 of $4.5 million with a man who accused Reverend Wayland Brown, a diocesan priest, of sexually abusing him in the 1980s.

He also attended school and parish celebrations throughout the diocese[3] Hartmayer was appointed to the board of directors of the National Catholic Educational Association in 2019.

"[16] When the Satanic Temple of Atlanta scheduled a black mass in that city in October 2024, Hartmayer announced possible legal action if the organizers planned to use consecrated host in the ceremony.

In June 2020, Hartmayer expressed his approval of the US Supreme Court decision that prevented the Trump Administration from terminating the Deferred Action for Childhood Arrivals (DACA) program for the children of undocumented immigrants.

He stated:The Catholic Church will continue to accompany our brothers and sisters on this journey and to pursue solutions to the complex issues of migration, immigration refugee resettlement and securing our borders.

[18] In November 2021, Hartmeyer commented on the conviction of three white men in the murder of Ahmaud Arbery, a black man, in Glynn County, Georgia:[19]“We are called to acknowledge and root out racism in our neighborhoods, schools, workplaces and yes, even our churches.

I hope you will join me in prayer, peace and restorative works of justice in this cause of respecting the human dignity of all.”[19]In 2023, Hartmayer participated in a ceremony with Jewish and Muslim religious leaders to commemorate the dead from the conflicts in Israel, Gaza and Ukraine.