The interior of Sweetest Heart of Mary Church is amazing, awe-inspiring, and artistic. It consists of a wide long nave flanked by side naves, a transept area, a sanctuary, and an extensive choir loft. The height of the vaulted ceiling at its highest point is seventy-six feet. Two rows of ten-inch wrought iron columns aid the walls in the support of the roof. With their ornamentation the columns are more than two feet in diameter. It may be interesting to note that these pillars, capped with extremely ornamental gold leafed Corinthian capitals (in which is ensconced indirect lighting) have the appearance of real marble. The fact is, however, that already in the early 1890s, they were very artistically covered with a hard plastic-like substance to simulate marble.

The lighting of the entire interior comes from the uniquely beautiful stained glass windows. In the wall area between these windows are found almost life-like Stations of the Cross which are extraordinarily beautiful statuary and give the interior a sense of balance. This interior was built to seat 2400 people. Being of such vast proportions is it any wonder that the Detroit newspapers of the I 890s referred to this grand church as "Kolasiñski's Cathedral"?

The superior vault of the nave is a series of groined sections, the arched ribs of the sections running into the center from the branching tops of the pillars. These centers are culminated by large gilded plaster rosettes. Though the original ceiling was painted in indigo and was relieved by a spattering of small gold stars, the entire vaulted ceiling was covered with a complex pattern of burnished gold, painted on canvas, in the times of Bishop Plagens. This rich golden design creates a striking and elegant effect. An exquisite border pattern outlines each of the numberless ribs in this vaulted ceiling. Tucked into the convergence of the ribs are paintings of angels. Interspersed among the large spaces of the upper reaches of the vaulting are medallions which feature portraits of the Virgin Mary and of many of the saints.