Cass and John Mayo were not just energetic operators of a furniture business. They were incessant entrepreneurs and civic promoters. Even before buying the land for the Mayo Building in 1909, Cass had built a new house a block west for his wife Allene, probably suspecting even then that the location would have a higher and better use ten or fifteen years later. Cass was elected to the board of the Commerce Club in 1908, the predecessor of Tulsa's Chamber of Commerce.
In 1915, shortly after he and John had added a second wing to the Mayo Building, Cass acquired 20 acres of land from the Perryman family in southeast Tulsa, an indication of things to come. Just two years after the brothers completed the Mayo Building’s second addition in 1918, Cass was working on plans to relocate the furniture company again, into the Petroleum Building, a new ten-story building they would build on the site of his former home. He was also investing in other real estate, notably the Oak Cliff subdivision on Reservoir Hill, which was run from the office next to his in the Mayo Building.
A vice president and director of the Atlas Life Insurance Company, Cass urged the company to build a new building on Boston Avenue. He was also a director of Central National Bank, on Main Street, which would later merge into the First National Bank of Tulsa. John would later serve on that bank's board for 40 years, during which they built two buildings on Boston for their expanding operations.
Across the street from the Petroleum Building, the Mayos owned the Halliburton-Abbott building, named for its clothing store tenant. This building was razed in the 1980s for the new Cities Service Building, which was acquired during construction and completed by ONEOK.
Cass continued to be a civic promoter, traveling to Washington with Tulsa booster trips, helping lead the subscription effort that built the Tulsa Municipal Airport, heading the Fair Board, serving as chairman of the streets sub-committee for the 1945 Citizens Bond Issue committee, and co-chairing a Hillcrest Hospital capital campaign. When he wasn’t working on business or civic projects he raised purebred Holstein-Friesian cattle and Belgian horses at the 1,880 acre Mayo farms on 51st Street, two miles east of Alsuma.
After Cass’s death in 1949, his son Allen (C.A. Mayo, Jr.) carried on the family businesses. In 1954, he undertook the renovation of the retail stores in the Mayo Building, giving the building the granite façade that it still presents at the street level. At 21st and Yale, he built the Mayo Meadows shopping center, on the southwest corner where the Walmart stands today. In the 1960s he sold part of the tract of land in front of his boyhood home to enable the development of the 2300, the apartment (and later, condominium) tower that overlooks the Arkansas River.