Main Campus

On November 4, 1923, Bishop Hartley purchased the original 20-acre site, with ground being broken for the main building in July 1924. By September 1925, construction of the main school building was completed and classes were held there. In 1931, the gymnasium and the Our Lady Lourdes Grotto were completed, and in 1941 the theater building on the west side of campus was completed.^[5] Major renovations were made to the gym in 1951 to more than double the capacity. A Gaelic-style chapel (called the "Lower Chapel") was added on to the east side of the school in 1937. Bishop Hartley dedicated the chapel to Mother of Mercy. A Milwaukee art company beautified the chapel with artwork in 1952. A natatorium was built next to the multipurpose room in 1990. In 1999, the Jack Ryan Training and Fitness Facility was built adjoining to the gym.

Construction on the US$5.5 million Robert C. Walter Student Commons and the Student Services and Fine Arts Center, the largest addition to Saint Charles in its history, began in June 2005. The approximately 27,000-square-foot (2,500 m²) addition, which replaced the courtyard behind the school, was ready for graduation at the end of the following school year; however, it was not available for full use until the 2006–2007 school year. The addition houses the 15,000-square-foot (1,400 m²) Robert C. Walter Student Commons Area, which serves as a new cafeteria and provides ample seating space for school and community functions. The Walter Student Commons is named in memory of Robert C. Walter, father of 1963 graduate Robert D. "Bob" Walter, honorary chairman and lead contributor to the current capital campaign being conducted to finance the new addition. Bob Walter and his '63 classmate, architect Robert Corna of Cleveland, initiated the concept for the Commons nearly two years ago. Corna was the architect on the project, and based his designs off of a similar plan for Saint Ignatius High School in Cleveland, also an all-male school. On the columns supporting the roof are glass panes, which bear the names of all of the alumni of the school, written with laser. Behind the Robert C. Walter Student Commons Area is the 12,000-square-foot (1,100 m²) Student Services & Fine Arts Center. The ground floor anchors the north end of the atrium, and houses a new kitchen, sponsored by Donatos Pizza, and restrooms. The second floor houses offices for the guidance and counseling programs, campus ministry, and the school nurse. On the third floor is an extensive art room and gallery, with a kiln and mud rooms; and a music and choir room, with instrument storage and practice rooms; as well as offices for the respective instructors. The second and third floor connect to the original building at its rear stairwell, and to the Robert C. Walter Student Commons Area by a staircase.

On November 1, 2018, Saint Charles announced the beginning of funding to its latest addition to main campus. The addition, once funded $20 million, will take the place of the current gymnasium and multi-purpose room and add additional classrooms, laboratories, and a new gymnasium. It is also planned that a hallway will connect the new addition to the main building. This new expansion will be on the current baseball field, and it is expected that Saint Charles will purchase a portion of a nearby park for use to build a new baseball diamond. On November 4, 2021, the school broke ground for its 11.5 million dollar Convocation Center named after its alumni Frank E. Murphy (who graduated in 1954).^[6]

In 2023, construction on the Brotherhood Center began. The Brotherhood Center will repurpose the school's former gymnasium into a multifaceted area with classrooms, offices, a library, and a new space that will help students with their academics. ^[7] As of March 2024, construction is still ongoing.

West Campus

For the first time in its 89-year history, the school expanded its current Broad Street campus footprint. The centerpiece of this project is the newly purchased property which formerly housed the Dealers Lumber Company and came up for sale in the summer of 2010. St. Charles moved quickly to acquire the 6.2-acre site and closed on the purchase in December. The Robert D. Walter West Campus includes the Savko Athletic Complex, comprising a six-lane running track, the artificial-turf Dominic and Kathleen Cavello Field and parking; a 13,000-square-foot training and fitness facility (weight room and 30 yard turf field for speed and agility); and the Horvath Parking Lot. A pedestrian bridge that spans Alum Creek helps connect it with the main campus at 2010 E. Broad St. The full project, paid for through fundraisers, private donations and in-kind gifts, cost about $5.1 million.

In 2018, Saint Charles unveiled the new Robotics and Mentoring Center (even further west from Main Campus than West Campus, thus gaining the colloquial "West West Campus" or "West-er Campus").^[8] The new center costs $2 million and is 14,000-square-feet.^[8] Along with being able to support the Robotics Team and My Brother's Keeper (MBK) mentoring programs, the new building has a recreational area for indoor sports, including an indoor basketball court and a state-of-the-art golf simulator.^[8] The new Mentoring Center is connected to West Campus via a sidewalk along Long Street.

Ohio State Championships

• Golf (Div I, 2)- 2009, 2010
• Soccer (A-AA, 2)- 1983, 1985
• Swimming (Div I, 1)- 2008
• Volleyball (non-OHSAA, 2)- 2009, 2018
• Water Polo (non-OHSAA, 7)- 2010, 2011, 2013, 2014, 2018, 2019, Spring 2021

The usual Fall 2020 water polo season was postponed to the spring of 2021 due to the Coronavirus pandemic; as such, the 2018, 2019, and Spring 2021 State Championships constitute a three-peat.