[1][2] Johnston has stated that the faith and generosity of his mother and the teachings of Pope John Paul II inspired him to enter the priesthood.

Johnston was named chancellor and moderator of the diocesan curia in 1996, and later pastor of Our Lady of Fatima Parish in Alcoa, Tennessee, in addition to his curial duties, on May 14, 2007.

[1] In 2002, Johnston and two other priests saved a man and his two children from falling over a waterfall in Glacier National Park in Montana.

"[1] Johnston supports the celebration of Tridentine Mass, but has said he does not "hold a great personal attachment to it" due to his relatively young age.

He later stated at a press conference, "I am eager to learn about the Church in Southern Missouri, and to become part of God's family here.

His predecessor, Bishop Robert Finn, had pleaded guilty in 2012 to a charge of failing to report an individual abusing a child.

[7] On March 23, 2021, Johnston announced the laicization in December 2020 of Reverend Michael Tierney, who had multiple credible accusations of sexual abuse of children.

In May 2009, noting that US President Barack Obama "has taken steps on multiple fronts to undermine the protection of innocent human life", Johnston said the decision of the University of Notre Dame to have Obama deliver the commencement speech at its graduation ceremony and receive an honorary degree was "saddening and bewildering.

In the letter, Johnston asked voters to examine which party supported so-called moral issues such as abortion rights.