Since 1995, Ernstrom has volunteered with Habitat for Humanity, where he works at the construction sites, trains and mentors new volunteers, and serves as chair of the building committee. Ernstrom is also a dedicated volunteer and supporter of his parish, St. Mary's in Los Gatos, where he serves as a lector, eucharistic minister, and RCIA sponsor. Ernstrom's daughters, Natalee '81 and Patricia '88, say their father's "character and commitment to lifelong learning, spirituality, and community service exemplify the Ignatian ideals central to Santa Clara's educational philosophy."