The Reverend Stephen Badin, who came to perform the first Catholic marriage in Fort Wayne, helped the local Catholics select the site that would become Cathedral Square. In 1831, for $100, they purchased a large portion of the present square from Samuel and Eliza Hanna. The first permanent pastor, the Reverend Louis Mueller, came in 1836, and the first Catholic church, St. Augustine's, was built on the property in 1837.

Reverend Julian Benoit, a French-born priest who replaced Mueller in 1840, had the greatest role in the development of the Cathedral. Popular with local citizens and the Miami Indians in the area, Benoit also served missionary stations throughout northeastern Indiana and northwest Ohio. He established the first Catholic school in Fort Wayne in 1845 with the Sisters of Providence and St. Mother Theodore Guerin.

When the Diocese of Fort Wayne was created by Pope Pius IX in 1857, Benoit marshaled supporters and finances - even from among local Protestants - to build a Cathedral to serve the diocese. The Right Reverend John H. Luers was its first bishop. Benoit already had purchased the remaining property in the block-square area, so St. Augustine's was moved to the east side of the square to make way for the Cathedral, and construction was started in 1859. St. Augustine's burned shortly after it was moved in 1857.

Benoit was co-architect with Thomas Lau and was involved in all aspects of the construction. The Cathedral cornerstone was placed on June 19, 1859, and the Cathedral of the Immaculate Conception was dedicated on December 8, 1860. Named to the National Register of HIstoric Places in 1980, the church "is believed to be the oldest building in continuous public use in northern Indiana."

Originally made of red brick trimmed with gray sandstone, the building, without furnishings, cost $54,000. Sometime around the early part of this century the sandstone brick exterior started to deteriorate and a stucco coating was used to cover it. The Cathedral was later refaced with Indiana limestone and Wisconsin lannonstone, a project that was postponed until after World War II and completed in 1950. The MacDougal Memorial Chapel and the Diocesan Chancery were constructed at the same time. French Gothic architecture was changed to English Gothic.

The present stained-glass windows were installed in 1896. Made at the Royal Bavarian Art Institute in Munich, Germany, the windows depict scenes in the life of Mary. The Stations of the Cross (also made in Germany) and the communion rail were installed the same time. The Chandelier lights handing over the pews date to 1928 to 1932. The large mission crucifix hanging at the front right of the pews was in St. Augustine's. The altar stone carried by Catholic missionaries to this area before being placed in St. Augustine's was moved to the Cathedral, where it has been part of the permanent altar since 1859.

Reverend Benoit is buried in the crypt below the high altar, along with two other rectors and 4 bishops - Right Reverend John Luers, Right Reverend Joseph Dwenger, Right Reverend Joseph Rademacher and Right Reverend Herman Alerding. Another burial site believed to be at Cathedral Square is that of Miami Chief Jean Baptiste de Richardville.

In 1924 Msgr. Thomas Conroy, who is sometimes called the "Rebuilder of the Cathedral," began extensive modifications to the building. He installed the renowned hand-carved sanctuary reredos and the bishop's chair. The reredos depict the redemption of mankind and church history. The figures are done in "grotesque Gothic" and are set in Gothic turrets. In 1960 the Diocese of Fort Wayne was redesignated by the Vatican as the Diocese of Fort Wayne- South Bend. It included 14 counties of northern and northeastern Indiana. In 1944 the southern counties of the original diocese were assigned to the new Diocese of Lafayette-in-Indiana. In 1957 the four northwest counties were assigned to the newly established Diocese of Gary.

In 1982 the Diocese of Fort Wayne-South Bend celebrated the 125th anniversary of its establishment on September 22. A historical marker was erected on Cathedral Square to commemorate the event. In 1986 the Cathedral Parish celebrated its sesquicentennial (1836-1986)