While at Fontbonne University, Cassilly met and married his first wife, painter and printmaker Cecelia Davidson. In May 1972, the couple honeymooned in Rome. They were visiting St. Peter's Basilica in Vatican City when Laszlo Toth attacked Michelangelo's The Pietà. Cassilly was the first to act and subdued Toth. While living in St. Louis, Cassilly and Davidson restored over 36 dilapidated Victorian buildings. These restorations led to the construction of six townhouses, for which he designed the architectural flourishes. The project led Cassilly to start making sculptures professionally. He soon became known for his public pieces that depict animals such as turtles and hippos. The couple also built and ran a restaurant in Lafayette Square. Eventually, they sold the restaurant, which allowed them to move to Hawaii, where Cassilly carved wooden figures. Cassilly reportedly grew tired of Hawaii, returned to his native St. Louis, and earned a master's degree in art at his alma mater, Fontbonne. There, he met sculptor Gail Soliwoda, whose works include the limestone monument at the Myron and Sonya Glassberg Family Conservation Area. Cassilly divorced Davidson and married Soliwoda. Cassilly and Soliwoda became business partners. In 1993, they bought a complex, which included the International Shoe Building, offices and a 10-story warehouse, for 69 cents per square foot. They renovated the site and opened it in 1997 as the City Museum, helping to spark a renovation boom in downtown St. Louis. The museum includes a shoelace factory, a fire truck, two airplanes, and a Ferris wheel on the roof. The Project for Public Spaces listed the museum among the "Great Public Spaces in the World" in 2005. In 2002, financial obligations forced Cassilly to begin charging visitors a fee to park at the museum. Cassilly hung a sign in the museum's parking lot reading, "Greedy Bob’s Parking Lot." Cassilly's other works include hippopotamus statues installed at Hippo Playground in Manhattan's Riverside Park in 1993. In 1997, Cassilly also contributed hippo sculptures to Central Park's Safari Playground near W. 91 Street. He designed two turtles for Turtle Park in St. Louis. A giant concrete butterfly, called the Mysterious Monarch, was unveiled in Faust Park outside the Butterfly House, Missouri Botanical Garden in 1997 in Chesterfield, Missouri. Cassilly's giraffe statue, which stands at the entrance to the Dallas Zoo, is the tallest sculpture in Texas at 67½ feet tall. His works for the St. Louis Zoo include the Sea Lion Fountains and a 45-foot squid statue. In 2000, Cassilly began work on Cementland, a repurposing of a former cement factory on a site in north St. Louis. In 2002, Cassilly and Soliwoda divorced.
On September 26, 2011, Cassilly died at Cementland. A police investigation initially found that he died of injuries after the bulldozer he was driving flipped down a hill. Further investigation showed that he was beaten to death, then staged to appear to be an accident. Cassily was survived by his third wife, Melissa Giovanna Cassilly, and their two children, Dylan and Robert III; and two children from his second marriage, Daisy and Max.
