Together with his wife, Catherine, Mullen envisioned the founding of a high school in Denver for orphaned boys.

In 1928, working with Henry Tihen, Mullen contacted Edward Flanagan, the founder of Boys Town, for advice on how best to design and operate such a school.

Following Flanagan's recommendation, Mullen wrote to the Christian Brothers of St. John Baptist de La Salle in Santa Fe, New Mexico, and invited them to be the directors and teachers of his planned school.

In June 1928, Mullen opened negotiations with the De La Salle Christian Brothers.

However, both Catherine and John Mullen died before the project could be completed, but their daughters and their husbands carried on with their plans.